Decoupling method for internal resistance of battery

Abstract

The invention provides a battery internal resistance decoupling method. A three-electrode battery is subjected to an EIS test; potential changes of the positive electrode, the negative electrode and the total battery in the discharging or charging process are monitored at the same time to form a direct-current pulse diagram; fitting EIS data obtained by testing by adopting an equivalent circuit model and utilizing Nova software to obtain R < Omega > and R < ct > of the total battery, the negative electrode and the positive electrode; calculating a concentration polarization pressure difference delta U in the direct current pulse diagram; respectively calculating Rdiff of the total battery, the negative electrode and the positive electrode through a formula; according to the method, a three-electrode method is adopted, electrodes in an equilibrium state are detected under constant potential through small-amplitude sine disturbance, an equivalent circuit diagram is constructed, and therefore R < Omega > and R < ct > are obtained. And according to the conversion relation between the frequency domain and the time domain, obtaining the Rct measurement ending time. The voltage corresponding to the time is found in the direct current pulse diagram, the voltage difference between the voltage and the voltage at the end of the pulse is concentration polarization, and the concentration polarization is divided by the pulse current to obtain Rdiff.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and particularly to a method for decoupling battery internal resistance. Background Art

[0002] During the operation of a battery, when current flows through the battery internally, it encounters resistance, resulting in voltage drop and polarization phenomena. Polarization mainly includes three types: ohmic polarization, electrochemical polarization, and concentration polarization. Ohmic polarization is caused by the ohmic internal resistance (R Ω ) of the battery, showing a linear relationship between current and voltage; electrochemical polarization is caused by the charge transfer impedance (R ct ) and is related to the kinetic resistance of the electrode reaction; concentration polarization is caused by the diffusion impedance (R diff ) and is related to the concentration change of reactants or products. These three types of polarization jointly affect the actual voltage output, energy efficiency, and service life of the battery, and are key factors to be considered in the performance optimization of the battery at low temperature and high rate. If the proportion of each part of polarization in the full battery can be known, targeted improvement can be carried out.

[0003] Therefore, we propose a method for decoupling battery internal resistance to understand the proportion of each part of polarization in the battery. Summary of the Invention

[0004] The purpose of the present invention is to solve the disadvantages existing in the prior art. To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] A method for decoupling battery internal resistance includes the following steps:

[0006] S1: Perform EIS test on a three-electrode battery;

[0007] S2: After the tested battery is left to stand for 1 - 5 min, discharge it with current I for 5 - 15 s, then leave it to stand for 1 - 5 min, charge it with current I for 5 - 15 s, and then leave it to stand for 1 - 5 min. During this period, simultaneously monitor the potential changes of the positive electrode, negative electrode, and full battery during the discharge or charge process to form a DC pulse diagram;

[0008] S3: Use an equivalent circuit model to fit the EIS data obtained from the test by Nova software to obtain R of the full battery, negative electrode, and positive electrode Ω and R ct ;

[0009] S4: Calculate the concentration polarization pressure difference ΔU in the DC pulse diagram; then calculate and obtain R of the full battery, negative electrode, and positive electrode respectively through the formula diff .

[0010] Further preferably, step S4 is to obtain the frequency f corresponding to the turning point of the arc and the oblique line from the data fitted in S3, and according to the formula calculate the corresponding time T; read the voltage U1 corresponding to the time T and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the concentration polarization pressure difference ΔU by U2 - U1.

[0011] Further preferably, the formula in step S4 is

[0012] Further preferably, step S4 is to read the voltage U0 of the initial data point and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the polarization pressure difference ΔU by U2 - U0.

[0013] Further preferably, the formula in step S4 is and R diff =R cell -R Ω -R ct , and calculate and obtain the R of the full cell, the negative electrode and the positive electrode respectively diff .

[0014] Further preferably, the test frequency range in S1 is 10 kHz to 10 mHz, and the test amplitude is 3 to 5 mV.

[0015] Further preferably, the three electrodes include a positive working electrode, a negative working electrode and a reference electrode.

[0016] Further preferably, the working electrode is a lithium iron phosphate positive electrode, the counter electrode is a graphite negative electrode, and the reference electrode is a lithium-plated copper wire.

[0017] Further preferably, the working electrode is a graphite negative electrode, the counter electrode is a lithium iron phosphate positive electrode, and the reference electrode is a lithium-plated copper wire.

[0018] Further preferably, the working electrode is a lithium iron phosphate positive electrode, the counter electrode is a graphite negative electrode, and the reference electrode is a graphite negative electrode.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] The present invention adopts the three-electrode method, detects the electrode in the equilibrium state under a constant potential through a small-amplitude sinusoidal perturbation, constructs an equivalent circuit diagram, and thus obtains R Ω and R ct . Then, according to the conversion relationship between the frequency domain and the time domain, obtain the time when the R ct measurement ends (i.e., the start of the R diff measurement). Find the voltage corresponding to this time in the DC pulse diagram, and the pressure difference between this voltage and the voltage at the end of the pulse is the concentration polarization, and then divide it by the pulse current to obtain Rdiff 。

[0021] By introducing a third electrode as a reference electrode, the present invention can independently monitor the electrochemical behaviors of the positive and negative electrodes, thereby realizing the decomposition of the internal resistance of the full cell into the positive and negative electrodes; by converting between the frequency domain and the time domain, the electrochemical impedance spectroscopy (EIS) test method is combined with the direct current pulse method (DCR) test method to further improve the current method for decoupling the internal resistance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the EIS Nyquist diagram of a 3.5 Ah three-electrode battery;

[0023] Figure 2 is the equivalent circuit model diagram of the three-electrode battery;

[0024] Figure 3 is the time-voltage curve diagram of a 3.5 Ah full cell;

[0025] Figure 4 is the time-voltage curve diagram of the negative electrode of a 3.5 Ah battery;

[0026] Figure 5 is the time-voltage curve diagram of the positive electrode of a 3.5 Ah battery;

[0027] Figure 6 is the schematic diagram of the correspondence between the electrochemical impedance spectroscopy and the direct current pulse method;

[0028] Figure 7 is the EIS Nyquist diagram of a 23 Ah three-electrode battery;

[0029] Figure 8 is the time-voltage curve diagram of a 23 Ah three-electrode battery. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0031] Referring to Figures 1-8 , the working principle and process of the present invention:

[0032] The following relationship exists between the frequency domain and the time domain of the electrochemical impedance spectroscopy:

[0033]

[0034] where f is the frequency (Hz) and T is the time (s).

[0035] Resistance calculation:

[0036]

[0037] Among them, ΔU is the voltage difference (V), I is the current (A), and R is the resistance (Ω).

[0038] 3. Resistance decoupling formula:

[0039] R cell = R Ω + R ct + R diff (Formula 3)

[0040] Example 1:

[0041] S1: Perform EIS test on the three - electrode battery;

[0042] In the example: Take a 3.5 Ah three - electrode lithium - ion soft - pack battery as an example. Use lithium - plated copper wire as the reference electrode, and graphite and lithium iron phosphate as the working electrode and the counter electrode respectively to perform EIS test. The frequency range is from 10 kHz to 10 mHz, and the amplitude is 3 - 5 mV.

[0043] S2: After the tested battery is left standing for 1 - 5 min, discharge it with current I for 5 - 15 s, then leave it standing for 1 - 5 min, charge it with current I for 5 - 15 s, and then leave it standing for 1 - 5 min. During this period, simultaneously monitor the potential changes of the positive electrode, negative electrode, and full battery during the discharge or charge process to form a DC pulse diagram;

[0044] S3: Use an equivalent circuit model to fit the EIS data obtained from the test using Nova software to obtain the R Ω and R ct ;

[0045] In the example: First, use graphite as the working electrode, the counter electrode is lithium iron phosphate, and the reference electrode is lithium - plated copper wire to perform EIS test ( Figure 1 the blue line Anode in Figure 1 ). Then swap the graphite and lithium iron phosphate electrodes, keep the reference electrode unchanged, and test EIS again ( Figure 1 the orange line Cathode in

[0046] ). Finally, use lithium iron phosphate as the working electrode, and both the counter electrode and the reference electrode are graphite electrodes, and test EIS again ( Figure 1 the yellow line Fullbattery in

[0046] ). Use the battery after the EIS test. After leaving it standing for 2 min, discharge it with a current value of 3.5 A for 10 s, leave it standing for 2 min again, charge it with a current value of 3.5 A (the current value can be customized) for 10 s, and leave it standing for 2 min again. During this period, simultaneously monitor the potential changes of the positive electrode, negative electrode, and full battery. Due to the reversibility of the charge - discharge process, only the discharge process or the charge process can be analyzed.

[0047] S4: Calculate the concentration polarization pressure difference ΔU in the DC pulse diagram; then, through the formula, calculate and obtain the Rs of the full cell, negative electrode, and positive electrode respectively diff .

[0048] In the embodiment: S4 includes two calculation methods. The formula in step S4 of method one is The calculation steps are as follows: Obtain the frequency f corresponding to the turning point of the arc and the straight line from the data fitted in S3. According to the formula calculate the corresponding time T; read the voltage U1 corresponding to time T and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the concentration polarization pressure difference ΔU through U2 - U1.

[0049] The calculation process of method one is:

[0050] In the Nova software, use the equivalent circuit model ( Figure 2 ) to fit the EIS data obtained from the test, and calculate and obtain the Rs of the full cell, negative electrode, and positive electrode respectively Ω and Rct (Table 2); calculation principle: where Rs Ω is the intersection point of the Nyquist diagram and the X-axis, and Rs ct is the diameter of the arc ( Figure 1 , Figure 6 ).

[0051] As Figure 1 , find the frequency f (such as 8 Hz) corresponding to the turning point of the arc and the straight line in the fitted data of the EIS. According to formula 1, calculate the corresponding time (such as 0.125 s). Find the voltage (U1) corresponding to this time in the DC pulse diagram. The pressure difference between this voltage and the voltage at the end of the pulse (U2) is the concentration polarization (U2 - U1) ( Figures 3-6 ). According to formula 2, then divide by the pulse current (such as 3.5 A) Note: Figures 3-5 All use the same calculation method to calculate and obtain the Rdiff of the full cell, negative electrode, and positive electrode respectively.

[0052] In the embodiment: S4 includes two calculation methods. The formula in step S4 of method two is and Rs diff = Rs cell - Rs Ω - Rs ct , and the calculation steps are as follows: Read the voltage U0 of the initial data point and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the polarization pressure difference ΔU through U2 - U0.

[0053] The calculation process of method two is:

[0054] As Figures 3-5 , read the voltage (U0) of the last data point shelved in the figure. The pressure difference between the voltage at the end of the pulse (U2) is the polarization (U2 - U0) of the battery at this SOC and this current (I). According to Equation 2, it is the internal resistance

[0055] According to Table 2, the R obtained by EIS Ω and R ct The sum of them is consistent with that calculated by DCR. Therefore, we can obtain R through EIS testing Ω and R ct , obtain R through DCR testing cell , and finally according to R cell =R Ω +R ct +R diff The R can be calculated diff .

[0056] Table 1: Voltage difference of 3.5 Ah battery

[0057]

[0058] Table 2: R of 3.5 Ah battery Ω , R ct and R diff

[0059]

[0060] Example 2:

[0061] S1: Perform EIS testing on the three - electrode battery;

[0062] In the example: Take a 23 Ah three - electrode lithium - ion soft - pack battery as an example. Use lithium - plated copper wire as the reference electrode, and graphite and lithium iron phosphate as the working electrode and the counter electrode respectively for EIS testing. The frequency range is from 10 kHz to 10 mHz, and the amplitude is 3 - 5 mV.

[0063] S2: After shelving the tested battery for 1 - 5 min, discharge it with current I for 5 - 15 s, then shelve it for 1 - 5 min, charge it with current I for 5 - 15 s, and then shelve it for 1 - 5 min. During this period, monitor the potential changes of the positive electrode, negative electrode, and full battery during the discharge or charge process to form a DC pulse diagram;

[0064] S3: Use an equivalent circuit model to fit the EIS data obtained by testing with Nova software to obtain R of the full battery, negative electrode, and positive electrode Ω and R ct ;

[0065] In the embodiment: First, graphite is used as the working electrode, the counter electrode is lithium iron phosphate, and the reference electrode is a lithium-plated copper wire for EIS testing ( Figure 7 the blue line Anode in it). Then, the graphite and the lithium iron phosphate electrode are swapped, the reference electrode remains unchanged, and EIS is tested again ( Figure 7 the orange line Cathode in it). Finally, lithium iron phosphate is used as the working electrode, and both the counter electrode and the reference electrode are graphite electrodes, and EIS is tested again ( Figure 7 the yellow line Fullbattery in it).

[0066] For the battery after EIS testing, after leaving it standing for 2 min, it is discharged at a current value of 23 A for 10 s, left standing for 2 min again, charged at a current value of 23 A (the current value can be customized) for 10 s, and left standing for 2 min again. During this period, the potential changes of the positive electrode, negative electrode, and full battery are monitored simultaneously. Due to the reversibility of the charge-discharge process, only the discharge process or the charge process can be analyzed.

[0067] S4: Calculate the concentration polarization voltage difference ΔU in the DC pulse diagram; then, through the formula, calculate and obtain the Rs of the full battery, negative electrode, and positive electrode respectively diff .

[0068] In the embodiment: S4 includes two calculation methods. The formula in step S4 of the first method is The calculation steps are as follows: Obtain the frequency f corresponding to the turning point of the arc and the straight line from the data fitted in S3, and calculate the corresponding time T according to the formula Read the voltage U1 corresponding to the time T and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the concentration polarization voltage difference ΔU through U2 - U1.

[0069] The calculation process of the first method is as follows: In the Nova software, the EIS data obtained from the test is fitted by using the equivalent circuit model ( Figure 2 ), and the Rs of the full battery, negative electrode, and positive electrode are calculated respectively Ω and R ct . The calculation principle: Among them, R Ω is the intersection point of the Nyquist diagram and the X-axis, and R ct is the diameter of the arc ( Figure 6 , Figure 7 ).

[0070] Find the frequency f (such as 5 Hz) corresponding to the turning point of the arc and the straight line in the fitted data of the EIS, and calculate the corresponding time (such as 0.2 s) according to formula 1. Find the voltage (U1) corresponding to this time in the DC pulse diagram, and the voltage difference between this voltage and the voltage (U2) at the end of the pulse is the concentration polarization (U2 - U1)( Figure 6 , Figure 8)。According to Equation 2, divide by the pulsed current (such as 3.5 A)

[0071] In the embodiment: S4 includes two calculation methods. The formula in step S4 of Method 2 is and R diff = R cell -R Ω -R ct , and its calculation steps are to read the voltage U0 of the initial data point and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the polarization voltage difference ΔU through U2 - U0.

[0072] The calculation process of Method 2 is as follows:

[0073] Such as Figure 8 is the time-voltage curve with a core capacity of 23 Ah, where: a. The time-voltage curve of the full cell; b. The partially enlarged view of the time-voltage curve of the full cell; c. The time-voltage curve of the positive electrode; d. The partially enlarged view of the time-voltage curve of the positive electrode; e. The time-voltage curve of the negative electrode; f. The partially enlarged view of the time-voltage curve of the negative electrode. Read the voltage (U0) of the last data point during rest in the figure, and the voltage difference from the voltage (U2) at the end of the pulse is the polarization (U2 - U0) of the battery at this SOC and this current (I), which is recorded in Table 3. According to Equation 2, it is the internal resistance

[0075] According to Table 4, the sum of R Ω and R ct obtained by EIS is consistent with that calculated by DCR. Therefore, we can obtain R Ω and R ct through EIS testing, and obtain R cell through DCR testing. Finally, according to R cell = R Ω + R ct + R diff , R diff can be calculated.

[0076] Table 3: Voltage difference of 23 Ah battery

[0077]

[0078] Table 4: R Ω , R ct and R diff

[0079]

[0080] The present invention accurately determines R by implanting three electrodes inside the battery cell, combining the conversion relationship between the intermediate frequency domain and the time domain of EIS, and using the DC pulse method. ct and R diff of the demarcation line.

[0081] The present invention can achieve accurate decoupling of R Ω , R ct and R diff in the full battery, and has higher accuracy and reliability compared with the traditional method.

[0082] The present invention adopts the three - electrode method, probes the electrode under the equilibrium state at a constant potential through small - amplitude sine perturbation, constructs an equivalent circuit diagram, and thus obtains R Ω and R ct . Then, according to the conversion relationship between the frequency domain and the time domain, the time when the measurement of R ct ends (i.e., the start of the measurement of R diff ) is obtained. The voltage corresponding to this time is found in the DC pulse diagram, and the pressure difference between this voltage and the voltage at the end of the pulse is the concentration polarization, and then divided by the pulse current, which is R diff .

[0083] Taking the lithium - ion battery system as an example, the present invention can separately monitor the electrochemical behaviors of the positive and negative electrodes by introducing a third electrode as a reference electrode, thereby realizing the decomposition of the internal resistance of the full battery into the positive and negative electrodes.

[0084] By combining the electrochemical impedance spectroscopy (EIS) test method with the DC pulse method (DCR) test method through frequency - domain and time - domain conversion, the current method for decoupling the internal resistance of the battery is further improved.

[0085] This method is applicable to the charge - discharge system under different current densities and has wide applicability.

[0086] It is further proved that R Ω and R ct are the intrinsic properties of the battery at this SOC and are independent of the magnitude of the applied current.

Claims

1. A decoupling method for battery internal resistance, characterized in that It includes the following steps: S1: Conduct an EIS test on the three-electrode battery; S2: After the tested battery is left standing for 1 - 5 minutes, it is discharged at current I for 5 - 15 seconds, then left standing for 1 - 5 minutes again, charged at current I for 5 - 15 seconds, and then left standing for 1 - 5 minutes again. During this period, the potential changes of the positive electrode, negative electrode, and full battery during the discharge or charge process are monitored simultaneously to form a DC pulse diagram; S3: Use the equivalent circuit model to fit the EIS data obtained from the test with Nova software to obtain the Rs of the full cell, the negative electrode, and the positive electrode Ω and Rs ct ; S4: Calculate the concentration polarization pressure difference ΔU in the DC pulse diagram; then, respectively calculate and obtain the Rs of the full cell, the negative electrode, and the positive electrode through the formula diff .

2. The decoupling method of battery internal resistance according to claim 1, wherein Step S4 is to obtain the frequency f corresponding to the turning point of the arc and the oblique line based on the data fitted in S3, and calculate the corresponding time T according to the formula ; read the voltage U1 corresponding to the time T and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the concentration polarization pressure difference ΔU by U2 - U1.

3. A decoupling method for the internal resistance of a battery according to claim 2, characterized in that, The formula in step S4 is 4. A decoupling method for the internal resistance of a battery according to claim 1, characterized in that, Step S4 is to read the voltage U0 of the initial data point and the voltage U2 corresponding to the end of the pulse in the DC pulse diagram, and calculate the polarization voltage difference ΔU through U2 - U0.

5. The decoupling method of battery internal resistance according to claim 4, characterized in that The formula in step S4 is and R diff = R cell - R Ω - R ct , and the Rs of the full cell, the negative electrode, and the positive electrode are calculated respectively diff .

6. A decoupling method for the internal resistance of a battery according to claim 1, characterized in that, In the S1, the test frequency range is 10 kHz - 10 mHz, and the test amplitude is 3 - 5 mV.

7. A decoupling method for the internal resistance of a battery according to claim 1, characterized in that, The three electrodes include a positive working electrode, a negative working electrode, and a reference electrode.

8. A decoupling method for battery internal resistance according to claim 7, characterized in that The working electrode is a lithium iron phosphate positive electrode, the counter electrode is a graphite negative electrode, and the reference electrode is a lithium-plated copper wire.

9. A decoupling method for the internal resistance of a battery according to claim 7, characterized in that The working electrode is a graphite negative electrode, the counter electrode is a lithium iron phosphate positive electrode, and the reference electrode is a lithium-plated copper wire.

10. A decoupling method for battery internal resistance according to claim 7, characterized in that The working electrode is a lithium iron phosphate positive electrode, the counter electrode is a graphite negative electrode, and the reference electrode is a graphite negative electrode.

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