Method for judging infiltration state of lithium ion battery

Abstract

The invention belongs to the field of battery testing, and particularly relates to a method for judging the infiltration state of a lithium ion battery, which comprises the following steps of: respectively carrying out electrochemical impedance spectroscopy (EIS) testing on a reference battery and a battery to be tested, carrying out DRT processing on the result, and comparing the maximum values of relaxation distribution functions gamma (tau) of the reference battery and the battery to be tested in a specific time constant tau range to judge the infiltration state of the lithium ion battery. And the infiltration state of the lithium ion battery to be detected is judged. According to the technical scheme, an innovative detection means based on the electrochemical impedance spectroscopy (EIS) is provided for challenges in monitoring the infiltration degree of the lithium ion battery and uncertainty of infiltration time control, and the purpose is to accurately evaluate the infiltration state of the lithium battery.

Description

Technical Field

[0001] The invention belongs to the field of battery testing, and in particular relates to a method for determining the infiltration state of a lithium-ion battery. Background Art

[0002] In lithium-ion batteries, sufficient infiltration of the battery electrode group by the electrolyte is a prerequisite for achieving high energy density and long cycle life of lithium-ion batteries. If the battery cell is not well infiltrated, it may lead to increased internal resistance of the battery, degradation of rate performance, shortened cycle life, and even the generation of lithium dendrites that threaten the safety of lithium batteries, which greatly restricts the performance and life of lithium-ion batteries. In view of this, in order to ensure the safe and reliable use of the battery, effective measures must be taken to check the infiltration state of the battery during the product development and production stage to ensure that it meets the qualified standards.

[0003] Among the current wetting state monitoring technologies, common methods include labeling, neutron diffraction, ultrasonic detection, and open circuit voltage measurement. However, each method has its own specific limitations. For example, the labeling method may change the original composition of the electrolyte by adding foreign substances to it, resulting in deviations in the measurement results; ultrasonic and neutron diffraction techniques have strict requirements on signal processing and rely on expensive professional equipment, which undoubtedly increases the cost burden of the production line; and the open circuit voltage method is also controversial due to the influence of many factors. Therefore, it is particularly urgent and critical to develop an economical and accurate technical means to evaluate the wetting condition of lithium battery electrode groups.

[0004] Patent CN114993888 discloses a method of using luminescent material marking to inspect the wetting state of lithium batteries. After injecting the electrolyte containing the luminescent material into the battery cell, the battery state is fixed with resin, and the distribution of the luminescent material in the battery cell is observed to determine the wetting state of the electrolyte. On the one hand, this method adds other organic substances (luminescent materials) to the electrolyte, which changes the state of the electrolyte and may cause deviation in the results; on the other hand, judging the wetting state by disassembling is a destructive detection method, which cannot achieve instant detection of the wetting state.

[0005] Patent CN1177881961A discloses a method for using neutron diffraction to test the wetting state of lithium batteries. Through the principle of interaction between neutrons and light elements such as H and Li, real-time monitoring of the battery wetting state is achieved. However, neutron diffraction-related equipment is expensive, and neutron diffraction has certain radiation damage to the human body, so it is not suitable for large-scale application.

[0006] Patent CN117740623A discloses a method for determining the electrolyte infiltration effect of a battery by detecting the change in open circuit voltage and AC impedance after battery injection. The principle of this method is that the open circuit potential and AC impedance of the battery will change regularly with the change in the electrode infiltration area of ​​the battery cell, but in actual applications, the open circuit voltage and AC impedance values ​​are affected by multiple factors such as temperature, pressure, injection volume, and infiltration state. The infiltration state is only one of the multiple influencing factors, so the accuracy of this method is controversial. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for determining the wetness state of a lithium ion battery.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for judging the wetting state of a lithium-ion battery comprises performing electrochemical impedance spectroscopy (EIS) tests on a reference battery and a battery to be tested respectively, performing DRT processing on the results, and comparing the maximum values ​​of relaxation distribution functions γ(τ) of the two under a specific time constant τ range to judge the wetting state of the lithium-ion battery to be tested.

[0010] The reference battery is a lithium ion battery of the same model as the battery to be tested, which has completed the injection process but has not undergone subsequent processes such as formation; the lithium ion battery is one of a lithium iron phosphate battery, a ternary lithium battery or a lithium manganese iron phosphate battery.

[0011] The battery to be tested is prepared in the following manner: the lithium-ion battery is charged with a short time and a small current after being filled with liquid, and then left to stand.

[0012] The charging current is 0.05C to 0.5C, preferably 0.1C; the charging time is 5min to 30min, preferably 10min.

[0013] The standing parameters are: standing time of 0h to 100h, and standing temperature of 20℃ to 80℃.

[0014] The electrochemical impedance spectroscopy (EIS) test specifically adopts the following steps: 1) Place the battery to be tested in a constant temperature oven at 20-45°C for 30-60 minutes to keep its temperature constant; 2) Use constant potential or constant current method to perform EIS measurement; 3) Perform Kramers-Kronig test on the obtained EIS results. If the test error of the impedance is ≤10%, the EIS result is considered valid and can be further analyzed; if the test error is >10%, it is necessary to adjust the parameters and retest.

[0015] In step 2), the voltage amplitude of the constant potential method is 2mV to 10mV, and the current amplitude of the constant current method is 5% to 10% of the rated 1C current of the lithium battery. The test frequency range is: 10 5 Hz~10 -2 Hz.

[0016] The analysis of E IS test results adopts the following steps: DRT processing of EIS results is performed using Mat lab or Python software;

[0017] In order to perform DRT processing on the EIS results of the battery, the expression of the total impedance Z(ω) of the battery is as follows:

[0018]

[0019] Where ω is the frequency, R0 is the ohmic impedance, γ(l nτ) is the relaxation time distribution, τ is the time constant, and j is the complex unit. The relaxation distribution function γ(τ) at different time constants τ in the EIS results is extracted through deconvolution technology.

[0020] The maximum values ​​of γ(τ) of the reference battery and the battery under test in the time constant range of 5s to 100s are denoted as γ 基准 and γ max , where the X-axis is the time constant τ and the Y-axis is the relaxation distribution function γ(τ).

[0021] The judgment criteria are: max When γ ≈ 0, it indicates that no infiltration has occurred or the degree of infiltration is extremely low; max <γ 基准 , it proves that the infiltration is not yet complete; γ max ≈γ 基准 When the infiltration is completed.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The technical solution of this application aims at the challenges of monitoring the degree of wetting of lithium-ion batteries and the uncertainty of controlling the wetting time, and proposes an innovative detection method based on electrochemical impedance spectroscopy (EIS), which aims to accurately evaluate the wetting state of lithium batteries. This method realizes the refined separation of EIS results from the perspective of time domain, effectively eliminating the potential interference of other irrelevant factors on the monitoring results; DRT technology is used to concretize the EIS impedance value, so as to accurately judge the degree of wetting of the battery. As a non-destructive method for detecting the degree of wetting, this method has guiding significance for the judgment of wetting time. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of evaluating the wetting state of a lithium-ion battery according to an embodiment of the present invention;

[0025] Figure 2 It is the reference benchmark diagram for DRT analysis in the implementation case;

[0026] Figure 3 This is a diagram showing the DRT analysis results for different infiltration times in the implementation case. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and the best embodiments.

[0028] Example 1

[0029] Determination of the degree of infiltration of lithium iron phosphate batteries (see flowchart for Figure 1 ):

[0030] 1. Perform ES I test on the reference battery to establish a reference benchmark and obtain γ 基准值 ;

[0031] The method comprises the following steps: selecting a lithium iron phosphate battery after liquid injection and sealing, wherein the battery capacity is 3.5Ah; performing an EIS test on the lithium iron phosphate battery, selecting a constant voltage mode for measurement, a voltage amplitude of 2mV, and a test frequency range of 10 5 Hz~10 -2 Hz; the obtained EIS results were subjected to Kramers-Kronig test, and if the test error of impedance was ≤10%, the EIS results were valid.

[0032] The above EIS test results are analyzed by DRT and plotted, and saved as a reference for completion of infiltration. The software used for data processing is Matlab or Python; the relaxation distribution function at different time constants in the EIS results is extracted by deconvolution technology. As shown in Figure 2, the maximum value of γ(τ) in the time constant τ range of 5s to 100s is γ 基准 , where the X-axis is the time constant and the Y-axis is the relaxation distribution function; the maximum value of γ(τ) in the range of 5s to 100s is about 630, denoted as γ 基准值 ≈630;

[0033] 2. Perform ESI test on the battery to be tested after infiltration to obtain γ max ; Specifically comprising the following steps: taking another lithium iron phosphate battery of the same model, charging it at a constant current of 0.35A (0.1C) for 10 minutes; standing it at room temperature (25°C) for 0h, 4h, 5h, 6h, 24h, 26h, and 28h respectively; performing EIS tests on the lithium-ion batteries standing for different lengths of time and analyzing the obtained DRT data. The test method and DRT data analysis method are the same as those of the reference battery, and obtaining γmax , the results are as follows Figure 3 Shown.

[0034] 3. Comparison max With γ 基准 Determine the infiltration status;

[0035] When the static time is 0h, γ max =0.63≈0, indicating that the infiltration has just begun;

[0036] When the static time is 4h, γ max =64<γ 基准 , indicating that infiltration is not complete;

[0037] When the static time is 5h, γ max =270<γ 基准 , indicating that infiltration is not complete;

[0038] When the static time is 6h, γ max =400<γ 基准 , indicating that infiltration is not complete;

[0039] When the static time is 24h, 26h and 28h, γ max Basically no longer changes, and γ max =600≈γ 基准 , indicating that infiltration is complete;

[0040] The results of disassembling the series of batteries are consistent with the results of this embodiment, which proves that the technical solution of this embodiment can be effectively applied to evaluate the infiltration state of battery electrolyte.

[0041] In summary, the technical solution of the present application aims at the challenges of monitoring the wetting degree of lithium-ion batteries and the uncertainty of controlling the wetting time, and proposes an innovative detection method based on electrochemical impedance spectroscopy (EIS), which aims to accurately evaluate the wetting state of lithium batteries. This method realizes the refined separation of EIS results from the perspective of time domain, effectively eliminating the potential interference of other irrelevant factors on the monitoring results; and uses DRT technology to concretize the EIS impedance value, so as to accurately judge the wetting degree of the battery. As a non-destructive wetting degree detection method, this method has guiding significance for the judgment of wetting time.

[0042] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for determining the wetness of a lithium-ion battery, characterized in that: The electrochemical impedance spectroscopy (EIS) test is performed on the reference battery and the battery to be tested respectively, and the results are processed by DRT. The maximum values ​​of the relaxation distribution function γ(τ) of the two under a specific time constant τ range are compared to determine the wetting state of the lithium-ion battery to be tested.

2. The method for determining the wetness of a lithium-ion battery according to claim 1, wherein: The reference battery is a lithium-ion battery of the same model as the battery to be tested, which has completed the injection and sealing process but has not undergone subsequent processes such as formation; the lithium-ion battery is one of a lithium iron phosphate battery, a ternary lithium battery or a lithium iron manganese phosphate battery.

3. The method for determining the wetting state of a lithium-ion battery according to claim 1, wherein: The battery to be tested is prepared in the following manner: the lithium-ion battery is charged with a short time and a small current after being filled with liquid, and then left to stand.

4. The method for determining the wetting state of a lithium-ion battery according to claim 3, characterized in that: The charging current is 0.05C to 0.5C, preferably 0.1C; the charging time is 5min to 30min, preferably 10min.

5. The method for determining the wetting state of a lithium-ion battery according to claim 3, characterized in that: The standing parameters are: standing time of 0h to 100h, and standing temperature of 20℃ to 80℃.

6. The method for determining the wetting state of a lithium-ion battery according to claim 1, wherein: The electrochemical impedance spectroscopy (EIS) test specifically adopts the following steps: 1) Place the battery to be tested in a constant temperature oven at 20-45°C for 30-60 minutes to keep its temperature constant; 2) Use constant potential or constant current method to perform EIS measurement; 3) Perform Kramers-Kronig test on the obtained EIS results. If the test error of impedance is ≤10%, the EIS result is considered valid and can be further analyzed; if the test error is >10%, it is necessary to adjust the parameters and retest.

7. The method for determining the wetting state of a lithium-ion battery according to claim 6, characterized in that: In step 2), the voltage amplitude of the constant potential method is 2mV to 10mV, and the current amplitude of the constant current method is 5% to 10% of the rated 1C current of the lithium battery. The test frequency range is: 10 5 Hz~10 -2 Hz.

8. The method for determining the wetting state of a lithium-ion battery according to claim 1, characterized in that: The following steps are used to analyze the EIS test results: DRT processing is performed on the EIS results using Matlab or Python software; the relaxation distribution function γ(τ) at different time constants τ in the EIS results is extracted using deconvolution technology.

9. The method for determining the wetting state of a lithium-ion battery according to claim 1, characterized in that: The maximum values ​​of γ(τ) of the reference battery and the battery under test in the time constant range of 5s to 100s are denoted as γ 基准 and γ max , where the X-axis is the time constant τ and the Y-axis is the relaxation distribution function γ(τ).

10. The method for determining the wetting state of a lithium-ion battery according to claim 9, characterized in that: The judgment criteria are: max When γ ≈ 0, it indicates that no infiltration has occurred or the degree of infiltration is extremely low; max <γ 基准 , it proves that the infiltration is not yet complete; γ max ≈γ 基准 When the infiltration is completed.

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