Lithium battery cell, power battery pulse self-heating method and condition determination method and device

Abstract

The present invention discloses a method and a device for determining pulse self-heating of a lithium battery cell and a power battery, and the method and the device for determining the pulse current frequency threshold value f min , obtain the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min and the minimum charging current I" when there is a risk of lithium plating at the negative electrode min ; Set the minimum discharge current I' min With the minimum charging current I" min The smaller value is used as the pulse current effective value threshold I max ; Thus, the pulse self-heating condition of lithium battery is determined. The pulse current frequency of lithium battery pulse self-heating is greater than or equal to f min , the effect of pulse self-heating of lithium battery on the life attenuation of lithium battery can be reduced. As long as the effective value of pulse current during pulse self-heating of lithium battery is less than I max , it can ensure that the lithium battery cell does not have side reactions such as SEI film decomposition and lithium precipitation, thereby reducing the impact of pulse self-heating on the life of the lithium battery cell and the life of the power battery.

Description

Technical Field

[0001] The present invention belongs to the field of power battery self-heating, and specifically relates to a lithium battery cell, a power battery pulse self-heating method, and a condition determination method and device. Background Art

[0002] Broadening the applicable temperature range of power batteries (lithium batteries) is the key to promoting their application. Due to the high viscosity of the electrolyte of lithium batteries at low temperatures, the low migration rate of lithium ions, the easy occurrence of lithium deposition during charging, and the low discharge energy efficiency, the application under low temperature conditions is severely limited. At present, the low-temperature heating of power batteries is mainly external heating, including gas heating, liquid heating, heating film heating, etc., which has low heating efficiency and increases the complexity of the spatial layout of the whole package. The new energy vehicle industry is increasingly inclined to adopt internal heating technology, that is, using the input of pulsed high-frequency alternating current to the lithium battery cell to generate ohmic heat inside the lithium battery cell, so as to quickly heat up the power battery. The self-heating efficiency of the lithium battery cell is related to the internal resistance value, specific heat capacity, and pulse current amplitude and frequency of the lithium battery cell itself. Generally, the smaller the pulse current frequency, the larger the pulse current, and the faster the heating temperature rise rate, but at the same time, it is more likely to cause overcharge and over-discharge inside the lithium battery cell, resulting in negative lithium deposition, SEI film decomposition, and even copper current collector dissolution. Therefore, how to determine the pulse self-heating conditions of lithium batteries (including pulse current frequency and pulse current size) to ensure that pulse self-heating does not affect the life and safety performance of lithium batteries, and then extend it to pulse self-heating of power batteries, is an important prerequisite for the promotion and application of this technology.

[0003] CN113991214A discloses a low-temperature lithium-ion battery AC heating method, which uses given design factor experiments to find the relationship between current, frequency and lithium precipitation, but cannot determine the boundary conditions, and does not identify the risk of SEI film decomposition under low SOC conditions, and does not consider the application of this technology under full SOC conditions.

[0004] CN104064836A discloses a low-temperature self-heating method for a lithium-ion battery, which describes a method for determining a frequency range that has no effect on the life of a lithium-ion battery by using an electrochemical impedance spectroscopy (EIS) test. However, the frequency range obtained by this method is relatively wide, including the electrochemical activation impedance response frequency range, and no optimal frequency range identification method is proposed, so the feasibility of practical application is poor. In addition, it also describes a method for confirming an optimal charging current value by using an upper boundary voltage and a lower boundary voltage. However, this method uses the battery usage boundary as the boundary voltage, which may not accurately correspond to the negative electrode overcharge lithium deposition potential and the over-discharge side reaction potential boundary, and cannot truly reflect the boundary points of side reactions such as negative electrode lithium deposition and SEI film decomposition. Summary of the invention

[0005] The purpose of the present invention is to provide a method and a method for determining the pulse self-heating of a lithium battery cell and a power battery, so as to reduce the influence of the pulse self-heating on the life of the lithium battery cell and the power battery.

[0006] The method for determining the pulse self-heating conditions of a lithium battery cell according to the present invention comprises:

[0007] Get the pulse current frequency threshold f min , obtain the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min and the minimum charging current I" when there is a risk of lithium plating at the negative electrode min .

[0008] The minimum discharge current I' min With the minimum charging current I" min The smaller value is used as the pulse current effective value threshold I max .

[0009] The pulse self-heating condition of lithium battery is determined as: f≥f min , I<I max .

[0010] Wherein, f is the pulse current frequency of the lithium battery cell during pulse self-heating, I is the effective value of the pulse current during pulse self-heating of the lithium battery cell, and the pulse current frequency threshold f is min The result is obtained by performing an electrochemical impedance spectroscopy test and analysis on the lithium battery cell at a first temperature, wherein the first temperature is a starting temperature of the lithium battery cell during pulse self-heating of the lithium battery cell.

[0011] Preferably, by selecting a plurality of different discharge currents, As a single discharge time, the starting SOC is equal to SOC min The lithium battery cell with three electrodes is subjected to multiple constant current discharge tests at the first temperature to obtain the minimum discharge current I' when there is a risk of decomposition of the negative electrode SEI film min Among them, SOC min is the minimum SOC of the lithium battery during pulse self-heating. As a single charge time, the starting SOC is equal to SOC max The lithium battery cell with three electrodes is subjected to multiple constant current charging tests at the first temperature to obtain the minimum charging current I" when there is a risk of lithium deposition at the negative electrode min Among them, SOC max It is the maximum SOC of the lithium battery during pulse self-heating.

[0012] by As a single discharge / charge time, a constant current discharge / charge test is performed on a lithium battery cell with three electrodes, and the boundary current of pulse discharge under low SOC conditions (i.e., the minimum discharge current I') can be obtained. min ) and the boundary current of pulse charging under high SOC (i.e., the minimum charging current I" min ). In order to ensure that the lithium battery cell does not have side reactions such as SEI film decomposition and lithium precipitation, whether in low SOC or high SOC conditions, I' min with I" min The smaller value is taken as the pulse current effective value threshold I max .

[0013] Preferably, if the negative electrode is Li / Li + The potential of the negative electrode (i.e., the negative electrode VS Li / L i+ potential) is greater than or equal to the SEI film decomposition potential, indicating that there is a risk of SEI film decomposition at the negative electrode. + The discharge current when the potential is equal to the SEI film decomposition potential is taken as the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min If the negative electrode is relative to Li / Li + The potential (i.e. negative electrode VS Li / L i+ potential) is less than or equal to the lithium deposition potential, indicating that there is a risk of lithium deposition at the negative electrode. + The charging current when the potential is equal to the lithium deposition potential is taken as the minimum charging current I" when there is a risk of lithium deposition at the negative electrode min The minimum discharge current I' obtained in this way min 、Minimum charging current I" min Under the premise of avoiding the influence of pulse self-heating on the life of lithium batteries, the range of the effective value of the pulse current during pulse self-heating of lithium batteries can be expanded, and more choices can be made for the actual effective value of the pulse current.

[0014] Preferably, the method of selecting multiple different discharge currents is: taking a preset discharge current i' as a starting point, and increasing the discharge current step by step according to a preset gradient; the method of selecting multiple different charging currents is: taking a preset charging current i" as a starting point, and increasing the charging current step by step according to a preset gradient. By taking i' as a starting point and increasing the discharge current step by step to realize the selection of multiple different discharge currents, the operation is convenient, and the minimum discharge current I' when there is a risk of decomposition of the negative electrode SEI film can be found relatively quickly. min By taking i" as the starting point and gradually increasing the charging current, multiple different charging currents can be selected. The operation is convenient, and the minimum charging current I" when there is a risk of negative electrode lithium deposition can be found relatively quickly. min .

[0015] Preferably, if the theoretical temperature rise rate does not need to be solved, an electrochemical impedance spectroscopy test and analysis of the lithium battery cell at the first temperature is performed to obtain the pulse current frequency threshold value f min The steps include:

[0016] The first step is to preset m test SOCs, which are SOC min , SOC2, ..., SOC m-1 , SOC max , and then proceed to step 2.

[0017] The second step is to place the lithium battery cell whose initial SOC is equal to the first test SOC (among the preset m test SOCs) in a preset low-temperature environment and let it stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low-temperature environment (that is, the lithium battery cell reaches thermal equilibrium), and then execute the third step; wherein the ambient temperature of the preset low-temperature environment is equal to the first temperature.

[0018] The third step is to perform an electrochemical impedance spectroscopy test on the lithium battery cell that has been left to rest (i.e., has reached thermal equilibrium), obtain a corresponding Bode plot, and then execute the fourth step.

[0019] Step 4: Analyze the Bode plot to obtain a frequency value when the reactance is equal to zero, and then execute step 5.

[0020] Step 5: Determine whether m frequency values ​​are obtained. If yes, execute step 7; otherwise, execute step 6.

[0021] Step 6: Place the lithium battery cell whose starting SOC is equal to the next test SOC (among the preset m test SOCs) in a preset low-temperature environment and let it stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low-temperature environment, and then return to execute step 3.

[0022] Step 7: Use the maximum frequency value among the m frequency values ​​as the pulse current frequency threshold value f min .

[0023] Preferably, if it is necessary to solve the theoretical temperature rise rate and find the optimal temperature monitoring point, an electrochemical impedance spectroscopy test and analysis of the lithium battery cell at the first temperature is performed to obtain the pulse current frequency threshold f min The steps include:

[0024] The first step is to preset m test SOCs, which are SOC min , SOC2, ..., SOC m-1 , SOC max , and then proceed to step 2.

[0025] The second step is to place the lithium battery cell whose initial SOC is equal to the first test SOC (among the preset m test SOCs) in a preset low-temperature environment and let it stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low-temperature environment (i.e., thermal equilibrium is reached inside the lithium battery cell), and then execute the third step; wherein the ambient temperature of the preset low-temperature environment is equal to the first temperature.

[0026] The third step is to perform an electrochemical impedance spectroscopy test on the lithium battery cell that has been left to rest (i.e., has reached thermal equilibrium), obtain the corresponding Nengquist diagram and Bode diagram, and then execute the fourth step.

[0027] The fourth step is to analyze the Bode plot to obtain a frequency value when the reactance is equal to zero, analyze the Nerquist plot to obtain a resistance when the reactance is equal to zero (since the activation resistance is equal to 0 when the reactance is equal to zero, the resistance is an ohmic resistance), correspond a frequency value when the reactance is equal to zero to a resistance (that is, correspond the frequency value to the resistance), and then execute the fifth step.

[0028] Step 5: Determine whether m frequency values ​​and m resistances are obtained. If yes, execute step 7; otherwise, execute step 6.

[0029] Step 6: Place the lithium battery cell whose starting SOC is equal to the next test SOC (among the preset m test SOCs) in a preset low-temperature environment and let it stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low-temperature environment, and then return to execute step 3.

[0030] Step 7: Use the maximum frequency value among the m frequency values ​​as the pulse current frequency threshold value f min , the resistance corresponding to the maximum frequency value among the m frequency values ​​is taken as the pulse current frequency threshold f min The corresponding ohmic resistance R ohm .

[0031] The pulse self-heating process of lithium battery cells should only produce Joule heat (that is, ohmic resistance response) and should not produce reaction heat (that is, no activation impedance response). Therefore, the frequency value when the reactance is equal to zero is selected as the safe frequency. min ~SOC max The maximum frequency value among the m frequency values ​​is taken as the pulse current frequency threshold f min .

[0032] Preferably, the lithium battery cells used in the second step and the sixth step are obtained by the following treatment:

[0033] At room temperature, the lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current is charged to the required test SOC. Discharging the lithium battery used for testing first and then charging it to the required test SOC can ensure a more accurate SOC value, thereby making the test results more accurate.

[0034] Preferably, the lithium battery cell with three electrodes used in the constant current discharge test is obtained by the following processing:

[0035] At room temperature, the three-electrode lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current charging to SOC min The three-electrode lithium battery used for testing was first discharged and then charged to SOC min , which can ensure that the SOC value is more accurate, and thus make the test results more accurate.

[0036] Preferably, the lithium battery cell with three electrodes used in the constant current charging test is obtained by the following processing:

[0037] At room temperature, the three-electrode lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current charging to SOC max The three-electrode lithium battery used for testing was first discharged and then charged to SOC max , which can ensure that the SOC value is more accurate, and thus make the test results more accurate.

[0038] Preferably, the above-mentioned method for determining the pulse self-heating conditions of the lithium battery cell further includes: determining the theoretical temperature rise rate during pulse heating of the lithium battery cell. Specifically:

[0039] Using the formula: Calculate the theoretical heat release Q (in J / s).

[0040] Using the formula: Calculate the theoretical temperature rise rate RR.

[0041] Among them, I1 is the effective value of a selected pulse current, I1<I max , m is the mass of the lithium battery (in kg), C p is the specific heat capacity of lithium battery (unit: J / (kg·℃)).

[0042] Calculating the theoretical heat release and the theoretical temperature rise rate, we can preliminarily determine the value of the pulse current with I1 as the effective value and f as the min As the pulse current frequency, the effect can be achieved by pulse self-heating of lithium batteries.

[0043] Preferably, the above-mentioned method for determining the pulse self-heating conditions of the lithium battery cell further includes: determining the optimal temperature monitoring point of the lithium battery cell.

[0044] Specifically:

[0045] Temperature sensors are arranged at the center of the large surface, the positive and negative ears, and the vicinity of the explosion-proof valve of the lithium battery cell whose starting temperature is the second temperature and the starting SOC is the first SOC as temperature monitoring points. min and less than or equal to SOC max .

[0046] I1 is taken as the effective value of the pulse current during the pulse self-heating of the lithium battery, and f min As the pulse current frequency during the pulse self-heating of the lithium battery cell, the lithium battery cell (that is, the lithium battery cell having the second temperature as the starting temperature and the first SOC as the starting SOC) is pulse self-heated.

[0047] According to the detection data of each temperature sensor, the actual temperature rise rate RR1 at the center of the large surface, the actual temperature rise rate RR2 at the positive and negative ears, and the actual temperature rise rate RR3 near the explosion-proof valve are calculated.

[0048] The optimal temperature monitoring point is selected based on the relationship between the actual temperature rise rate RR1 at the center of the large surface, the actual temperature rise rate RR2 at the positive and negative ears, and the actual temperature rise rate RR3 near the explosion-proof valve and the theoretical temperature rise rate RR as well as the operational convenience of the layout.

[0049] The selection of the optimal temperature monitoring point has a great influence on the pulse self-heating process of the lithium battery cell. Accurate selection of the temperature monitoring point is conducive to ensuring that the temperature detected during the pulse self-heating process of the lithium battery cell can reflect the actual temperature of the lithium battery cell, and is conducive to ensuring that the time when the pulse self-heating of the lithium battery cell starts and ends is accurate, avoiding excessive power consumption due to excessive heating of the lithium battery cell.

[0050] Preferably, the method of selecting the best temperature monitoring point is:

[0051] The actual temperature rise rate RR1 is subtracted from the theoretical temperature rise rate RR, and the absolute value is taken to obtain the first temperature rise rate difference corresponding to the center of the large surface; the actual temperature rise rate RR2 is subtracted from the theoretical temperature rise rate RR, and the absolute value is taken to obtain the second temperature rise rate difference corresponding to the positive and negative ears; the actual temperature rise rate RR3 is subtracted from the theoretical temperature rise rate RR, and the absolute value is taken to obtain the third temperature rise rate difference corresponding to the vicinity of the explosion-proof valve.

[0052] If there is only one minimum value among the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference, the temperature monitoring point corresponding to the minimum value is used as the optimal temperature monitoring point.

[0053] If there are two minimum values ​​among the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference: if the two minimum values ​​are the first temperature rise rate difference and the second temperature rise rate difference, respectively, the positive and negative pole ears are taken as the optimal temperature monitoring points; if the two minimum values ​​are the first temperature rise rate difference and the third temperature rise rate difference, respectively, the area near the explosion-proof valve is taken as the optimal temperature monitoring point; if the two minimum values ​​are the second temperature rise rate difference and the third temperature rise rate difference, respectively, the positive and negative pole ears are taken as the optimal temperature monitoring points.

[0054] If the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference are all equal, the positive and negative tabs are used as the optimal temperature monitoring points.

[0055] The temperature rise rate of the best temperature monitoring point should be close to the theoretical temperature rise rate, so the temperature monitoring point with the smallest difference from the theoretical temperature rise rate is selected as the best temperature monitoring point. If the temperature rise rates of two or three temperature monitoring points are close to the theoretical temperature rise rate, the best temperature monitoring point is selected based on the operational convenience of the temperature sensor layout.

[0056] The device for determining the pulse self-heating condition of a lithium battery cell of the present invention comprises a controller, and the controller is programmed to execute the method for determining the pulse self-heating condition of a lithium battery cell.

[0057] The lithium battery pulse self-heating method of the present invention comprises:

[0058] The pulse self-heating condition of the lithium battery cell is determined by using the above-mentioned method for determining the pulse self-heating condition of the lithium battery cell.

[0059] According to the heating rate requirement, the pulse current frequency and pulse current effective value that meet the pulse self-heating conditions of the lithium battery cell are selected.

[0060] Based on the selected pulse current frequency and pulse current effective value, pulse self-heating of the lithium battery cell is performed.

[0061] The method for determining the pulse self-heating condition of a power battery according to the present invention, wherein the power battery is composed of n identical lithium batteries connected together, comprises:

[0062] The above-mentioned method for determining the pulse self-heating condition of a lithium battery cell is used to determine the pulse self-heating condition of a lithium battery cell in a power battery.

[0063] The pulse self-heating condition of the power battery is determined according to the connection relationship of n lithium batteries in the power battery; specifically:

[0064] If n lithium cells are connected in series (i.e., the power battery is composed of n lithium cells connected in series), then based on the pulse self-heating condition of the lithium cell, the pulse self-heating condition of the power battery is determined as: z ≥f min , I z <I max .

[0065] If n lithium batteries are connected in parallel (i.e., the power battery is composed of n lithium batteries connected in parallel), then based on the pulse self-heating condition of the lithium battery, the pulse self-heating condition of the power battery is determined as: z ≥f min , I z <n*I max .

[0066] If n lithium batteries are connected in series (i.e., the power battery is composed of n lithium batteries connected in series; connected in series means connected in parallel), the pulse current effective value threshold I of the power battery is determined based on the pulse self-heating condition of the lithium battery and the connected in series structure of the n lithium batteries. z_max , and then determine the power battery pulse self-heating condition as: f z ≥f min , I z <I z_max .

[0067] Among them, f z is the pulse current frequency of the power battery during pulse self-heating, I z It is the effective value of the pulse current during pulse self-heating of the power battery.

[0068] The power battery pulse self-heating method of the present invention comprises:

[0069] The power battery pulse self-heating condition is determined by using the power battery pulse self-heating condition determination method.

[0070] According to the heating rate requirement, the pulse current frequency and pulse current effective value that meet the pulse self-heating conditions of the power battery are selected.

[0071] Based on the selected pulse current frequency and pulse current effective value, pulse self-heating of the power battery is performed.

[0072] The present invention has the following effects:

[0073] (1) The present invention uses electrochemical impedance spectroscopy to test and analyze to obtain the pulse current frequency threshold f min, thus determining the lower limit of the pulse current frequency during the pulse self-heating of the lithium battery. The pulse current frequency during the pulse self-heating of the lithium battery only needs to be greater than or equal to f min , it can ensure that there is no lithium ion deintercalation and intercalation behavior, thereby reducing the impact of lithium battery cell pulse self-heating on the life decay of lithium battery cells.

[0074] (2) Since the decomposition of SEI film will lead to irreversible loss of lithium, affecting the cycle life of lithium battery cells, lithium deposition may cause lithium dendrites to pierce the diaphragm, leading to the risk of thermal runaway; therefore, in order to ensure that the lithium battery cells do not experience side reactions such as SEI film decomposition and lithium deposition, whether in low SOC or high SOC conditions, the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition is taken min Minimum charging current I" when there is a risk of lithium deposition at the negative electrode min The smaller value is taken as the pulse current effective value threshold I max ; As long as the pulse current effective value of the lithium battery cell is less than I max , it can ensure that the lithium battery cell does not have side reactions such as SEI film decomposition and lithium precipitation, thereby avoiding the reduction of safety of pulse self-heating and reducing the impact of pulse self-heating on the life of lithium battery cells and power batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 This is a flow chart for determining the pulse self-heating conditions of the lithium battery in Example 1.

[0076] Figure 2 The pulse current frequency threshold f obtained in Example 1 min Flowchart of the process.

[0077] Figure 3 This is a flow chart for determining the pulse self-heating conditions of the lithium battery in Example 2.

[0078] Figure 4 The pulse current frequency threshold f obtained in Example 2 min Flowchart of the process.

[0079] Figure 5 The Bode diagrams of the lithium battery cells with an initial SOC of 90% in Examples 1 and 2 are shown.

[0080] Figure 6 This is the Energquist diagram of the lithium battery cell with an initial SOC of 90% in Example 2.

[0081] Figure 7 This is a flow chart for determining the optimal temperature monitoring point of the lithium battery cell in Example 2. DETAILED DESCRIPTION

[0082] Example 1: Figure 1As shown, a lithium battery cell with a rated capacity of 50Ah and a mass of m=0.9kg is used for testing. The method for determining the pulse self-heating condition of the lithium battery cell in this embodiment includes:

[0083] Step S1, obtaining the pulse current frequency threshold value f min , obtain the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min and the minimum charging current I" when there is a risk of lithium plating at the negative electrode min .

[0084] Pulse current frequency threshold f min The first temperature is obtained by performing an electrochemical impedance spectroscopy test and analysis on the lithium battery at the first temperature, wherein the first temperature is the starting temperature of the lithium battery during pulse self-heating of the lithium battery, and in this embodiment, the first temperature is equal to -25°C.

[0085] like Figure 2 As shown, the pulse current frequency threshold f is obtained min The specific methods include:

[0086] In the first step, 6 (i.e., m=6) test SOCs are preset, and then the second step is performed. The 6 test SOCs are SOC min =10%, SOC2=20%, SOC3=30%, SOC4=80%, SOC5=90%, SOC max =100%.

[0087] Step 2: Place the lithium battery cell with the initial SOC equal to the first test SOC (e.g. 10%) of the six test SOCs in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then proceed to step 3. The lithium battery cell here must be treated as follows: At room temperature (25±2℃), place the lithium battery cell with a rated capacity of 50Ah and a mass of m=0.9kg in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then proceed to step 3. The current is discharged to the discharge cut-off voltage, and then The current is charged to the first test SOC of 6 test SOCs.

[0088] Step 3: Perform an electrochemical impedance spectroscopy test on the lithium battery cell after it has been left to rest, obtain the corresponding Bode plot, and then proceed to step 4. The amplitude of the electrochemical impedance spectroscopy test is a constant voltage of 5mV and a frequency of 0.01Hz to 10kHz.

[0089] Step 4: Analyze the Bode plot to obtain a frequency value when the reactance is equal to zero, and then execute step 5. Figure 5 The Bode plot of a lithium battery with a starting SOC equal to 90% is shown. Figure 5 The horizontal axis is frequency and the vertical axis is reactance. Figure 5It can be seen that the frequency value when the reactance is equal to zero is 210Hz (that is, the horizontal coordinate of point b is 210Hz).

[0090] Step 5: Determine whether 6 frequency values ​​are obtained. If yes, execute step 7; otherwise, execute step 6.

[0091] Step 6: Place the lithium battery cell with the starting SOC equal to the next test SOC in the 6 test SOCs in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then return to step 3. The lithium battery cell here must be processed as follows: At room temperature (25±2℃), place the lithium battery cell with a rated capacity of 50Ah and a mass of m=0.9kg in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then return to step 3. The current is discharged to the discharge cut-off voltage, and then The current is charged to the next test SOC among the 6 test SOCs.

[0092] Step 7: Use the maximum frequency value among the 6 frequency values ​​as the pulse current frequency threshold f min .

[0093] After the above electrochemical impedance spectroscopy test and analysis, it is obtained that min =10% corresponds to a frequency value of 210Hz, SOC2 = 20% corresponds to a frequency value of 260Hz, SOC3 = 30% corresponds to a frequency value of 230Hz, SOC4 = 80% corresponds to a frequency value of 230Hz, SOC5 = 90% corresponds to a frequency value of 210Hz, and SOC max =100% corresponds to a frequency value of 250Hz. Therefore, 260Hz is selected as the pulse current frequency threshold f min , that is, f min =260Hz.

[0094] The minimum discharge current I' when there is a risk of SEI film decomposition at the negative electrode is obtained min The specific method is:

[0095] Select multiple different discharge currents to As a single discharge time, a three-electrode lithium battery with an initial SOC of 10% is subjected to multiple constant current discharge tests at -25°C. Specifically, the three-electrode lithium battery with an initial SOC of 10% is placed in a -25°C environment for 3 hours to reduce the temperature of the lithium battery to -25°C (i.e., the lithium battery reaches thermal equilibrium), and then a constant current discharge test is performed. The three-electrode lithium battery here must first be treated as follows: At room temperature (25±2°C), the three-electrode lithium battery with a rated capacity of 50Ah is placed at The current is discharged to the discharge cut-off voltage, and then The current is charged to 10%.

[0096] If the negative electrode is relative to Li / Li + The potential of the negative electrode (i.e., the negative electrode VS Li / L i+ The potential of the negative electrode relative to Li / Li + The discharge current when the potential is equal to the SEI film decomposition potential is taken as the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min 10% is the minimum SOC of the lithium battery cell during pulse self-heating. The method of selecting a plurality of different discharge currents is: taking a preset discharge current i' as a starting point, and increasing the discharge current step by step according to a preset gradient.

[0097] For example, in this embodiment, the preset discharge current i'=3C is used as the starting point, and the discharge current is gradually increased according to the preset gradient 1C. That is, first discharge at 3C for 3.8ms, and observe that the negative electrode VS Li / L i+ Potential <1V, then discharged at 4C for 3.8ms, and observed negative electrode VS Li / L i+ The potential is <1V, and then the discharge current is increased and discharged for 3.8ms. When the discharge current is increased to 12C, the negative electrode VS Li / L i+ Potential = 1V, then 12C is taken as the minimum discharge current I' when there is a risk of decomposition of the negative electrode SEI film min , i.e. I' min =12C.

[0098] Get the minimum charging current I" when there is a risk of negative electrode lithium deposition min The specific method is:

[0099] Select multiple different charging currents to As a single charge time, a lithium battery cell with three electrodes and an initial SOC equal to 100% is subjected to multiple constant current charging tests at -25°C. Specifically, the lithium battery cell with three electrodes and an initial SOC equal to 100% is placed in a -25°C environment for 3 hours to reduce the temperature of the lithium battery cell to -25°C (that is, the lithium battery cell reaches thermal equilibrium), and then a constant current charging test is performed. The lithium battery cell with three electrodes here must first be treated as follows: At room temperature (25±2°C), the lithium battery cell with a rated capacity of 50Ah and three electrodes is placed at The current is discharged to the discharge cut-off voltage, and then The current is charged to 100%.

[0100] If the negative electrode is relative to Li / Li + The potential of the negative electrode (i.e., the negative electrode VS Li / L i+The potential) is less than or equal to the lithium deposition potential (in this embodiment, the value is 0V), which indicates that there is a risk of lithium deposition at the negative electrode. + The charging current when the potential is equal to the lithium deposition potential is taken as the minimum charging current I" when there is a risk of lithium deposition at the negative electrode min . Among them, 100% is the maximum SOC of the lithium battery during pulse self-heating. The method of selecting multiple different charging currents is: taking the preset charging current i" as the starting point, and increasing the charging current step by step according to the preset gradient.

[0101] For example, in this embodiment, the preset charging current i"=3C is used as the starting point, and the charging current is gradually increased according to the preset gradient 1C. That is, first charge at 3C for 3.8ms, and observe that the negative electrode VS Li / L i+ Potential>0V, then charged at 4C for 3.8ms, and observed negative electrode VS Li / L i+ Potential>0V, then continue to increase the charging current and charge for 3.8ms. When the charging current increases to 9C, it is observed that the negative electrode VS Li / L i+ Potential = 0V, then 9C is used as the minimum charging current I" when there is a risk of lithium deposition at the negative electrode min , that is I" min =9C.

[0102] Step S2: Determine the pulse current effective value threshold I max Specifically, the minimum discharge current I' when there is a risk of decomposition of the negative electrode SEI film min Minimum charging current I" when there is a risk of lithium deposition at the negative electrode min The smaller value is used as the pulse current effective value threshold I max In this embodiment, the pulse current effective value threshold I max =9C.

[0103] Step S3: Determine the pulse self-heating condition of the lithium battery: f≥f min , I<I max Wherein, f is the pulse current frequency during the pulse self-heating of the lithium battery, and I is the effective value of the pulse current during the pulse self-heating of the lithium battery. That is, the pulse self-heating conditions of the lithium battery in this embodiment are: the pulse current frequency f≥260Hz, and the pulse current effective value I<9C.

[0104] The device for determining the pulse self-heating condition of a lithium battery cell in this embodiment includes a controller, and the controller is programmed to execute the above-mentioned method for determining the pulse self-heating condition of a lithium battery cell.

[0105] The lithium battery pulse self-heating method in this embodiment includes:

[0106] The first step is to determine the pulse self-heating conditions of the lithium battery using the above-mentioned method for determining the pulse self-heating conditions of the lithium battery.

[0107] The second step is to select the pulse current frequency and pulse current effective value that meet the pulse self-heating conditions of the lithium battery cell according to the heating rate requirements.

[0108] The third step is to perform pulse self-heating of the lithium battery cell based on the selected pulse current frequency and pulse current effective value.

[0109] The power battery in this embodiment is composed of n identical lithium cells connected together.

[0110] The method for determining the pulse self-heating conditions of a power battery includes:

[0111] First, the pulse self-heating condition of a lithium battery cell in the power battery is determined by using the above-mentioned method for determining the pulse self-heating condition of the lithium battery cell.

[0112] Then, the pulse self-heating condition of the power battery is determined according to the connection relationship of the n lithium cells in the power battery. Specifically:

[0113] If n lithium batteries are connected in series, based on the above lithium battery pulse self-heating conditions, the power battery pulse self-heating conditions are determined as: z ≥f min , I z <I max .

[0114] If n lithium batteries are connected in parallel, based on the above lithium battery pulse self-heating conditions, the power battery pulse self-heating condition is determined as: z ≥f min , I z <n*I max .

[0115] If n lithium batteries are connected in series (in series and in parallel), the pulse current effective value threshold I of the power battery is determined based on the above-mentioned lithium battery pulse self-heating condition and the mixed structure of n lithium batteries. z_max , and then determine the power battery pulse self-heating condition as: f z ≥f min , I z <I z_max .

[0116] The power battery pulse self-heating method in this embodiment includes:

[0117] The first step is to determine the power battery pulse self-heating condition by using the above power battery pulse self-heating condition determination method.

[0118] The second step is to select the pulse current frequency and pulse current effective value that meet the above-mentioned power battery pulse self-heating conditions according to the heating rate requirements.

[0119] The third step is to perform pulse self-heating of the power battery based on the selected pulse current frequency and pulse current effective value.

[0120] Example 2: Figure 3 As shown, a lithium battery cell with a rated capacity of 50Ah and a mass of m=0.9kg is used for testing. The method for determining the pulse self-heating condition of the lithium battery cell in this embodiment includes:

[0121] Step S1, obtaining the pulse current frequency threshold value f min , obtain the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min and the minimum charging current I" when there is a risk of lithium plating at the negative electrode min .

[0122] Pulse current frequency threshold f min The first temperature is obtained by performing an electrochemical impedance spectroscopy test and analysis on the lithium battery at the first temperature, wherein the first temperature is the starting temperature of the lithium battery during pulse self-heating of the lithium battery, and in this embodiment, the first temperature is equal to -25°C.

[0123] like Figure 4 As shown, the pulse current frequency threshold f is obtained min The specific methods include:

[0124] In the first step, 6 (i.e., m=6) test SOCs are preset, and then the second step is performed. The 6 test SOCs are SOC min =10%, SOC2=20%, SOC3=30%, SOC4=80%, SOC5=90%, SOC max =100%.

[0125] Step 2: Place the lithium battery cell with the initial SOC equal to the first test SOC (e.g. 10%) of the six test SOCs in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then proceed to step 3. The lithium battery cell here must be treated as follows: At room temperature (25±2℃), place the lithium battery cell with a rated capacity of 50Ah and a mass of m=0.9kg in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then proceed to step 3. The current is discharged to the discharge cut-off voltage, and then The current is charged to the first test SOC of 6 test SOCs.

[0126] Step 3: Perform electrochemical impedance spectroscopy test on the lithium battery cell after standing to obtain the corresponding Nengquist plot and Bode plot, and then proceed to step 4. The amplitude of the electrochemical impedance spectroscopy test is a constant voltage of 5mV and the frequency is: 0.01Hz~100kHz.

[0127] The fourth step is to analyze the Bode plot to obtain a frequency value when the reactance is equal to zero, analyze the Nerquist plot to obtain a resistance when the reactance is equal to zero (since the activation resistance is equal to 0 when the reactance is equal to zero, the resistance is an ohmic resistance), correspond a frequency value when the reactance is equal to zero to a resistance (that is, correspond the frequency value to the resistance), and then execute the fifth step. Figure 5 The Bode plot of a lithium battery with a starting SOC equal to 90% is shown. Figure 5 The horizontal axis is frequency and the vertical axis is reactance. Figure 6 The Energquist diagram of a lithium battery with a starting SOC equal to 90% is shown. Figure 6 The horizontal axis is resistance (including ohmic resistance and activation resistance), and the vertical axis is reactance. Figure 5 It can be seen that the frequency value when the reactance is equal to zero is 210Hz (that is, the horizontal coordinate of point b is 210Hz). Figure 6 It can be seen that the resistance when the reactance is zero is 1.49mΩ (that is, the horizontal coordinate of point c is 1.49mΩ).

[0128] Step 5: Determine whether 6 frequency values ​​and 6 resistances are obtained. If yes, execute step 7; otherwise, execute step 6.

[0129] Step 6: Place the lithium battery cell with the starting SOC equal to the next test SOC in the 6 test SOCs in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then return to step 3. The lithium battery cell here must be processed as follows: At room temperature (25±2℃), place the lithium battery cell with a rated capacity of 50Ah and a mass of m=0.9kg in a -25℃ environment for 3 hours to reduce the temperature of the lithium battery cell to -25℃ (i.e. the lithium battery cell reaches thermal equilibrium), and then return to step 3. The current is discharged to the discharge cut-off voltage, and then The current is charged to the next test SOC among the 6 test SOCs.

[0130] Step 7: Use the maximum frequency value among the 6 frequency values ​​as the pulse current frequency threshold f min , the resistance corresponding to the maximum frequency value among the 6 frequency values ​​is taken as the pulse current frequency threshold f min The corresponding ohmic resistance R ohm .

[0131] After the above electrochemical impedance spectroscopy test and analysis, it is obtained that min=10% corresponds to a frequency value of 210Hz and a resistance of 1.51mΩ, and SOC2 = 20% corresponds to a frequency value of 260Hz and a resistance of 1.48mΩ, and SOC3 = 30% corresponds to a frequency value of 230Hz and a resistance of 1.45mΩ, and SOC4 = 80% corresponds to a frequency value of 230Hz and a resistance of 1.46mΩ, and SOC5 = 90% corresponds to a frequency value of 210Hz and a resistance of 1.49mΩ, and SOC max =100% corresponds to a frequency value of 250Hz and a resistance of 1.48mΩ. Therefore, 260Hz is selected as the pulse current frequency threshold f min , that is, f min =260Hz, then the pulse current frequency threshold f min The corresponding ohmic resistance R ohm =1.48mΩ.

[0132] The minimum discharge current I' when there is a risk of SEI film decomposition at the negative electrode is obtained min The specific method is:

[0133] Select multiple different discharge currents to As a single discharge time, a three-electrode lithium battery with an initial SOC of 10% is subjected to multiple constant current discharge tests at -25°C. Specifically, the three-electrode lithium battery with an initial SOC of 10% is placed in a -25°C environment for 3 hours to reduce the temperature of the lithium battery to -25°C (i.e., the lithium battery reaches thermal equilibrium), and then a constant current discharge test is performed. The three-electrode lithium battery here must first be treated as follows: At room temperature (25±2°C), the three-electrode lithium battery with a rated capacity of 50Ah is placed at The current is discharged to the discharge cut-off voltage, and then The current is charged to 10%.

[0134] If the negative electrode is relative to Li / Li + The potential of the negative electrode (i.e., the negative electrode VS Li / L i+ The potential of the negative electrode relative to Li / Li + The discharge current when the potential is equal to the SEI film decomposition potential is taken as the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min 10% is the minimum SOC of the lithium battery cell during pulse self-heating. The method of selecting a plurality of different discharge currents is: taking a preset discharge current i' as a starting point, and increasing the discharge current step by step according to a preset gradient.

[0135] For example, in this embodiment, the preset discharge current i'=3C is used as the starting point, and the discharge current is gradually increased according to the preset gradient 1C. That is, first discharge at 3C for 3.8ms, and observe that the negative electrode VS Li / L i+ Potential <1V, then discharged at 4C (3.8ms, observed negative electrode VS Li / L i+ The potential is <1V, and then the discharge current is increased and discharged for 3.8ms. When the discharge current is increased to 12C, the negative electrode VS Li / L i+ Potential = 1V, then 12C is taken as the minimum discharge current I' when there is a risk of decomposition of the negative electrode SEI film min , i.e. I' min =12C.

[0136] Get the minimum charging current I" when there is a risk of negative electrode lithium deposition min The specific method is:

[0137] Select multiple different charging currents to As a single charge time, a lithium battery cell with three electrodes and an initial SOC equal to 100% is subjected to multiple constant current charging tests at -25°C. Specifically, the lithium battery cell with three electrodes and an initial SOC equal to 100% is placed in a -25°C environment for 3 hours to reduce the temperature of the lithium battery cell to -25°C (that is, the lithium battery cell reaches thermal equilibrium), and then a constant current charging test is performed. The lithium battery cell with three electrodes here must first be treated as follows: At room temperature (25±2°C), the lithium battery cell with a rated capacity of 50Ah and three electrodes is placed at The current is discharged to the discharge cut-off voltage, and then The current is charged to 100%.

[0138] If the negative electrode is relative to Li / Li + The potential (i.e. negative electrode VS Li / L i+ The potential) is less than or equal to the lithium deposition potential (in this embodiment, the value is 0V), which indicates that there is a risk of lithium deposition at the negative electrode. + The charging current when the potential is equal to the lithium deposition potential is taken as the minimum charging current I" when there is a risk of lithium deposition at the negative electrode min . Among them, 100% is the maximum SOC of the lithium battery during pulse self-heating. The method of selecting multiple different charging currents is: taking the preset charging current i" as the starting point, and increasing the charging current step by step according to the preset gradient.

[0139] For example, in this embodiment, the preset charging current i"=3C is used as the starting point, and the charging current is gradually increased according to the preset gradient 1C. That is, first charge at 3C for 3.8ms, and observe that the negative electrode VS Li / L i+Potential>0V, then charged at 4C for 3.8ms, and observed negative electrode VS Li / L i+ Potential>0V, then continue to increase the charging current and charge for 3.8ms. When the charging current increases to 9C, it is observed that the negative electrode VS Li / L i+ Potential = 0V, then 9C is used as the minimum charging current I" when there is a risk of lithium deposition at the negative electrode min , that is I" min =9C.

[0140] Step S2: Determine the pulse current effective value threshold I max Specifically, the minimum discharge current I' when there is a risk of decomposition of the negative electrode SEI film min Minimum charging current I" when there is a risk of lithium deposition at the negative electrode min The smaller value is used as the pulse current effective value threshold I max In this embodiment, the pulse current effective value threshold I max =9C.

[0141] Step S3: Determine the pulse self-heating condition of the lithium battery: f≥f min , I<I max Wherein, f is the pulse current frequency during the pulse self-heating of the lithium battery, and I is the effective value of the pulse current during the pulse self-heating of the lithium battery. That is, the pulse self-heating conditions of the lithium battery in this embodiment are: the pulse current frequency f≥260Hz, and the pulse current effective value I<9C.

[0142] Step S4, determining the theoretical temperature rise rate of the lithium battery during pulse heating. Specifically:

[0143] First use the formula: Calculate the theoretical heat release Q (in J / s).

[0144] Then, using the formula: Calculate the theoretical temperature rise rate RR.

[0145] Among them, I1 is the effective value of a selected pulse current, I1<I max , m = 0.9 kg, C p is the specific heat capacity of the lithium battery. In this embodiment, I1 = 3C, taking SOC2 = 20% as an example, the theoretical heat release Q = (3*50) 2 *0.00148=33.3J / s, theoretical temperature rise rate RR=2.46℃ / min.

[0146] Step S5: Determine the optimal temperature monitoring point of the lithium battery. Figure 7 As shown, specifically:

[0147] In the first step, temperature sensors are arranged at the center of the large surface, the positive and negative ears, and near the explosion-proof valve of the lithium battery cell with a starting temperature of -25°C and a starting SOC of 20% as temperature monitoring points, and then the second step is performed.

[0148] The second step is to use 3C as the effective value of the pulse current during pulse self-heating of the lithium battery cell, and 260Hz as the pulse current frequency during pulse self-heating of the lithium battery cell, perform pulse self-heating on the lithium battery cell, and then execute the third step.

[0149] The third step is to calculate the actual temperature rise rate RR1 at the center of the large surface, the actual temperature rise rate RR2 at the positive and negative ears, and the actual temperature rise rate RR3 near the explosion-proof valve based on the detection data of each temperature sensor, and then execute the fourth step.

[0150] Step 4: Select the best temperature monitoring point based on the relationship between the actual temperature rise rate RR1 at the center of the large surface, the actual temperature rise rate RR2 at the positive and negative ears, and the actual temperature rise rate RR3 near the explosion-proof valve and the theoretical temperature rise rate RR, as well as the convenience of layout operation. The best temperature monitoring point is selected as follows:

[0151] Subtract the actual temperature rise rate RR1 from the theoretical temperature rise rate RR, and take the absolute value to obtain the first temperature rise rate difference corresponding to the center of the large surface; subtract the actual temperature rise rate RR2 from the theoretical temperature rise rate RR, and take the absolute value to obtain the second temperature rise rate difference corresponding to the positive and negative ears; subtract the actual temperature rise rate RR3 from the theoretical temperature rise rate RR, and take the absolute value to obtain the third temperature rise rate difference corresponding to the vicinity of the explosion-proof valve.

[0152] If there is only one minimum value among the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference, the temperature monitoring point corresponding to the minimum value is used as the optimal temperature monitoring point.

[0153] If there are two minimum values ​​among the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference: if the two minimum values ​​are the first temperature rise rate difference and the second temperature rise rate difference, respectively, the positive and negative pole ears are taken as the optimal temperature monitoring points; if the two minimum values ​​are the first temperature rise rate difference and the third temperature rise rate difference, respectively, the vicinity of the explosion-proof valve is taken as the optimal temperature monitoring point; if the two minimum values ​​are the second temperature rise rate difference and the third temperature rise rate difference, respectively, the positive and negative pole ears are taken as the optimal temperature monitoring points.

[0154] If the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference are all equal, the positive and negative ears are taken as the optimal temperature monitoring points.

[0155] In this embodiment, the temperature rise rate of the center of the large surface of the lithium battery cell is calculated to be 1.97°C / min, the temperature rise rate at the positive and negative pole ears of the lithium battery cell is 4.75°C / min, and the temperature rise rate near the explosion-proof valve of the lithium battery cell is 2.33°C / min. Therefore, the best temperature monitoring point is near the explosion-proof valve of the lithium battery cell.

[0156] The device for determining the pulse self-heating condition of a lithium battery cell in this embodiment includes a controller, and the controller is programmed to execute the above-mentioned method for determining the pulse self-heating condition of a lithium battery cell.

[0157] The lithium battery cell pulse self-heating method, the power battery pulse self-heating condition determination method and the power battery pulse self-heating method in this embodiment are the same as those in Embodiment 1.

Claims

1. A method for determining pulse self-heating conditions of a lithium battery, characterized in that: include: Get the pulse current frequency threshold f min , obtain the minimum discharge current I' when there is a risk of negative electrode SEI film decomposition min and the minimum charging current I" when there is a risk of lithium plating at the negative electrode min ; The minimum discharge current I' min With the minimum charging current I" min The smaller value is used as the pulse current effective value threshold I max ; The pulse self-heating condition of lithium battery is determined as: f≥f min , I<I max ; Wherein, f is the pulse current frequency during the pulse self-heating of the lithium battery, and I is the effective value of the pulse current during the pulse self-heating of the lithium battery; Get the pulse current frequency threshold f min The method is: select m starting SOCs equal to SOC min To SOC max The lithium battery cell is subjected to an electrochemical impedance spectroscopy test at a first temperature to obtain corresponding m Bode diagrams; the m Bode diagrams are analyzed to obtain m frequency values ​​when the reactance is equal to zero, and the maximum frequency value among the m frequency values ​​is used as the pulse current frequency threshold value f min ; Get the minimum discharge current I' min The method is: by selecting multiple different discharge currents, As a single discharge time, the starting SOC is equal to SOC min The lithium battery cell with three electrodes is subjected to multiple constant current discharge tests at the first temperature, and the negative electrode is taken relative to Li / Li + The discharge current when the potential is equal to the SEI film decomposition potential is taken as the minimum discharge current I' min ; Get the minimum charging current I" min The method is: by selecting multiple different charging currents, As a single charge time, the starting SOC is equal to SOC max The lithium battery cell with three electrodes is subjected to multiple constant current charging tests at the first temperature, and the negative electrode is taken relative to Li / Li + The charging current when the potential is equal to the lithium deposition potential is taken as the minimum charging current I" min ; Among them, SOC min is the minimum SOC of the lithium battery during pulse self-heating, SOC max is the maximum SOC of the lithium battery cell during pulse self-heating, and the first temperature is the starting temperature of the lithium battery cell during pulse self-heating.

2. The method for determining pulse self-heating conditions of a lithium battery according to claim 1, characterized in that: If the negative electrode is relative to Li / Li + If the potential of the negative electrode is greater than or equal to the SEI film decomposition potential, it means that there is a risk of SEI film decomposition at the negative electrode; if the negative electrode is relative to Li / Li + If the potential is less than or equal to the lithium plating potential, it means there is a risk of lithium plating at the negative electrode.

3. The method for determining pulse self-heating conditions of a lithium battery according to claim 1, characterized in that: The method of selecting a plurality of different discharge currents is: taking a preset discharge current i' as a starting point, and increasing the discharge current step by step according to a preset gradient; The method of selecting a plurality of different charging currents is: taking a preset charging current i" as a starting point, and increasing the charging current step by step according to a preset gradient.

4. The method for determining pulse self-heating conditions of a lithium battery according to any one of claims 1 to 3, characterized in that: Get the pulse current frequency threshold f min The steps include: The first step is to preset m test SOCs, which are SOC min , SOC2, ..., SOC m-1 , SOC max , then execute the second step; Step 2: placing the lithium battery cell with the initial SOC equal to the first test SOC in a preset low temperature environment and leaving it to stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low temperature environment, and then executing step 3; wherein the ambient temperature of the preset low temperature environment is equal to the first temperature; Step 3: Perform an electrochemical impedance spectroscopy test on the lithium battery cell after it has been left to stand, obtain the corresponding Bode diagram, and then proceed to step 4; Step 4: Analyze the Bode plot to obtain a frequency value when the reactance is equal to zero, and then execute step 5; Step 5: Determine whether m frequency values ​​are obtained. If yes, execute step 7; otherwise, execute step 6. Step 6: Place the lithium battery cell whose initial SOC is equal to the next test SOC in a preset low temperature environment and let it stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low temperature environment, and then return to step 3; Step 7: Use the maximum frequency value among the m frequency values ​​as the pulse current frequency threshold value f min .

5. The method for determining pulse self-heating conditions of a lithium battery according to claim 4, characterized in that: The lithium battery cells used in the second and sixth steps are obtained by the following treatments: At room temperature, the lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then The current is charged to the required test SOC; The lithium battery cell with three electrodes used in the constant current discharge test is obtained by the following processing: At room temperature, the three-electrode lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current charging to SOC min ; The lithium battery cell with three electrodes used in the constant current charging test is obtained by the following processing: At room temperature, the three-electrode lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current charging to SOC max .

6. The method for determining pulse self-heating conditions of a lithium battery according to any one of claims 1 to 3, characterized in that: Get the pulse current frequency threshold f min The steps include: The first step is to preset m test SOCs, which are SOC min , SOC2, ..., SOC m-1 , SOC max , then execute the second step; Step 2: placing the lithium battery cell with the initial SOC equal to the first test SOC in a preset low temperature environment and leaving it to stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low temperature environment, and then executing step 3; wherein the ambient temperature of the preset low temperature environment is equal to the first temperature; Step 3: Perform an electrochemical impedance spectroscopy test on the lithium battery cell after it has been left to stand, obtain the corresponding Nengquist diagram and Bode diagram, and then proceed to step 4; Step 4: Analyze the Bode diagram to obtain a frequency value when the reactance is equal to zero, analyze the Nerquist diagram to obtain a resistance when the reactance is equal to zero, correspond a frequency value when the reactance is equal to zero to a resistance, and then execute step 5; Step 5: Determine whether m frequency values ​​and m resistances are obtained. If yes, execute step 7; otherwise, execute step 6. Step 6: Place the lithium battery cell whose initial SOC is equal to the next test SOC in a preset low temperature environment and let it stand until the temperature of the lithium battery cell is equal to the ambient temperature of the preset low temperature environment, and then return to step 3; Step 7: Use the maximum frequency value among the m frequency values ​​as the pulse current frequency threshold value f min , the resistance corresponding to the maximum frequency value among the m frequency values ​​is taken as the pulse current frequency threshold f min The corresponding ohmic resistance R ohm .

7. The method for determining pulse self-heating conditions of a lithium battery according to claim 6, characterized in that: The lithium battery cells used in the second and sixth steps are obtained by the following treatments: At room temperature, the lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then The current is charged to the required test SOC; The lithium battery cell with three electrodes used in the constant current discharge test is obtained by the following processing: At room temperature, the three-electrode lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current charging to SOC min ; The lithium battery cell with three electrodes used in the constant current charging test is obtained by the following processing: At room temperature, the three-electrode lithium battery used for testing The current is discharged to the discharge cut-off voltage, and then Current charging to SOC max .

8. The method for determining pulse self-heating conditions of a lithium battery according to claim 6, characterized in that: The determination method also includes: determining the theoretical temperature rise rate during pulse heating of the lithium battery cell; specifically: Using the formula: Calculate the theoretical heat release Q; Using the formula: Calculate the theoretical temperature rise rate RR; Among them, I1 is the effective value of a selected pulse current, I1<I max , m is the mass of lithium battery, C p is the specific heat capacity of lithium battery.

9. The method for determining pulse self-heating conditions of a lithium battery according to claim 8, characterized in that: The determination method further includes: determining the optimal temperature monitoring point of the lithium battery cell; specifically: Temperature sensors are arranged at the center of the large surface, the positive and negative ears, and the vicinity of the explosion-proof valve of the lithium battery cell whose starting temperature is the second temperature and the starting SOC is the first SOC as temperature monitoring points; wherein the second temperature is less than or equal to the first temperature, and the first SOC is greater than or equal to the SOC min and less than or equal to SOC max ; I1 is taken as the effective value of the pulse current during pulse self-heating of the lithium battery, and f min As the pulse current frequency during pulse self-heating of the lithium battery cell, the lithium battery cell is pulse self-heated; According to the detection data of each temperature sensor, the actual temperature rise rate RR1 at the center of the large surface, the actual temperature rise rate RR2 at the positive and negative ears, and the actual temperature rise rate RR3 near the explosion-proof valve are calculated; The optimal temperature monitoring point is selected based on the relationship between the actual temperature rise rate RR1 at the center of the large surface, the actual temperature rise rate RR2 at the positive and negative ears, and the actual temperature rise rate RR3 near the explosion-proof valve and the theoretical temperature rise rate RR as well as the operational convenience of the layout.

10. The method for determining pulse self-heating conditions of a lithium battery cell according to claim 9, characterized in that: The best way to select the temperature monitoring point is: Subtract the actual temperature rise rate RR1 from the theoretical temperature rise rate RR, and take the absolute value to obtain a first temperature rise rate difference corresponding to the center of the large surface; subtract the actual temperature rise rate RR2 from the theoretical temperature rise rate RR, and take the absolute value to obtain a second temperature rise rate difference corresponding to the positive and negative ears; subtract the actual temperature rise rate RR3 from the theoretical temperature rise rate RR, and take the absolute value to obtain a third temperature rise rate difference corresponding to the vicinity of the explosion-proof valve; If there is only one minimum value among the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference, then the temperature monitoring point corresponding to the minimum value is used as the optimal temperature monitoring point; If there are two minimum values ​​among the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference: if the two minimum values ​​are the first temperature rise rate difference and the second temperature rise rate difference, respectively, the positive and negative pole ears are taken as the optimal temperature monitoring points; if the two minimum values ​​are the first temperature rise rate difference and the third temperature rise rate difference, respectively, the area near the explosion-proof valve is taken as the optimal temperature monitoring point; if the two minimum values ​​are the second temperature rise rate difference and the third temperature rise rate difference, respectively, the positive and negative pole ears are taken as the optimal temperature monitoring points; If the first temperature rise rate difference, the second temperature rise rate difference, and the third temperature rise rate difference are all equal, the positive and negative tabs are used as the optimal temperature monitoring points.

11. A device for determining pulse self-heating conditions of a lithium battery cell, comprising a controller, characterized in that: The controller is programmed to perform the determination method according to any one of claims 1 to 10.

12. A lithium battery pulse self-heating method, characterized in that: include: Determine the pulse self-heating condition of the lithium battery cell using the determination method according to any one of claims 1 to 10; According to the heating rate requirement, the pulse current frequency and pulse current effective value that meet the pulse self-heating conditions of the lithium battery cell are selected; Based on the selected pulse current frequency and pulse current effective value, pulse self-heating of the lithium battery cell is performed.

13. A method for determining pulse self-heating conditions of a power battery, wherein the power battery is composed of n identical lithium batteries connected together, characterized in that: The determination method includes: Determine the pulse self-heating condition of a lithium battery cell in a power battery using the determination method according to any one of claims 1 to 10; The pulse self-heating condition of the power battery is determined according to the connection relationship of n lithium batteries in the power battery; specifically: If n lithium batteries are connected in series, based on the pulse self-heating condition of the lithium battery, the pulse self-heating condition of the power battery is determined as: z ≥f min , I z <I max ; If n lithium battery cells are connected in parallel, based on the pulse self-heating condition of the lithium battery cell, the pulse self-heating condition of the power battery is determined as: z ≥f min , I z <n*I max ; If n lithium batteries are connected in series, the pulse current effective value threshold I of the power battery is determined based on the pulse self-heating condition of the lithium battery and the connection structure of the n lithium batteries. z_max , and then determine the power battery pulse self-heating condition as: f z ≥f min , I z <I z_max ; Among them, f z is the pulse current frequency of the power battery during pulse self-heating, I z It is the effective value of the pulse current during pulse self-heating of the power battery.

14. A pulse self-heating method for a power battery, characterized in that: include: Determine the pulse self-heating condition of the power battery by using the determination method as claimed in claim 13; According to the heating rate requirement, the pulse current frequency and pulse current effective value that meet the pulse self-heating conditions of the power battery are selected; Based on the selected pulse current frequency and pulse current effective value, pulse self-heating of the power battery is performed.

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