Resonant ultrasound spectroscopy-based nondestructive testing apparatus and method for lithium battery

Abstract

A resonant ultrasound spectroscopy-based nondestructive testing apparatus and method for a lithium battery. The apparatus comprises a sweep-frequency signal generation module, a power amplification module, a transmitting ultrasonic transducer, a receiving ultrasonic transducer, a pre-amplification module, a data acquisition module and a lock-in amplification module. The testing method comprises: amplifying the power of a sweep-frequency signal to serve as an excitation signal of a transmitting transducer, and using same to perform lock-in processing on a signal received by a receiving transducer, so as to obtain transmitted and reflected wave-based resonant ultrasound spectra at different positions of a lithium battery; identifying a plurality of resonance frequencies from the resonant ultrasound spectra, and analyzing resonant ultrasound spectrum features and the passband and stopband features within a resonant ultrasound spectrum signal; extracting from the resonant ultrasound spectrum signal amplitude information of transmitted waves at different frequency points to obtain an amplitude cloud map of the transmitted waves of the lithium battery at the plurality of frequency points and a mechanical quality factor of the lithium battery; and detecting a failure state of the lithium battery, and quantitatively evaluating the SoC of the lithium battery.

Description

A device and method for nondestructive testing of lithium batteries based on ultrasonic resonance spectrum

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from and is based on a Chinese patent application filed with the State Intellectual Property Office of China on August 14, 2023, with application number: 202311018433.5, and the name of the invention: "A device and method for non-destructive testing of lithium batteries based on ultrasonic resonance spectrum". The content of the Chinese patent application is hereby introduced into the text of this application. Technical Field

[0003] The present invention belongs to the technical field of lithium battery detection, and particularly relates to a device and method for non-destructive detection of lithium batteries based on ultrasonic resonance spectroscopy. Background Art

[0004] Lithium-ion batteries are electrochemical energy storage devices with the best comprehensive performance to date, and have been widely used in 3C electronic products, new energy vehicles, and other mobile electronic devices. During the production and use of lithium-ion batteries, safety or performance failures such as wrinkling, gas production, poor wetting, lithium precipitation, and extrusion damage are prone to occur. Among them, the coating of the inner core electrode and the wrinkling of the diaphragm will lead to uneven distribution of internal resistance of the battery, which will affect the cycle performance of the lithium battery. During the formation, overcharge, over-discharge, and storage process, lithium batteries may produce gases such as hydrogen, carbon monoxide, and carbon dioxide. If the gas inside the lithium battery reaches a certain amount, it will not only affect the performance of the lithium battery, but also pose a major hidden danger to the battery safety. Studies have shown that lithium-ion batteries often produce a certain amount of gas in the early stages of thermal runaway. Therefore, monitoring the gas inside the lithium battery provides an effective method for thermal runaway early warning monitoring. The infiltration of the electrolyte coating on the electrode in a lithium battery involves three-phase contact between solid, liquid, and gas. When the electrolyte is injected into the lithium battery shell, it displaces the air inside and simultaneously enters the space between the positive electrode, separator, and negative electrode, adhering to the positive and negative electrode surfaces. If the infiltration time is long enough, the electrolyte infiltration of the electrode will reach an equilibrium state under the action of surface tension. During the lithium battery formation process, areas with poor electrolyte infiltration have difficulty forming a protective solid-liquid interface film or the quality of the formed film is poor, which seriously affects the battery's cycle performance. In addition, the electrode not exposed to the electrolyte cannot participate in the electrochemical reaction, resulting in increased interfacial resistance, causing battery capacity decay. In severe cases, lithium plating can occur, increasing the risk of thermal runaway in the battery cell. Lithium battery failure can not only lead to a significant decrease in overall battery performance, but also pose significant safety risks such as combustion or explosion. Therefore, monitoring and evaluating the state of health (SoH) of lithium-ion batteries is a key part of lithium battery management. Furthermore, a lithium battery's state of charge (SoC), also known as residual capacity, refers to the ratio of the remaining capacity of a lithium battery after a period of use or long-term unused state to its rated capacity under the same conditions. Lithium battery SoC assessment is also a key component of lithium battery management.

[0005] Existing non-destructive testing and evaluation technologies for lithium batteries include X-ray imaging technology, magnetic field scanning imaging technology, ultrasonic testing technology, neutron diffraction technology, terahertz time-domain spectroscopy technology, etc. Among them, ultrasonic testing technology is highly valued due to its low cost and no potential risk of injury to operators. At present, the existing ultrasonic non-destructive testing technology for lithium batteries mainly includes the following testing methods: (1) Pulsed ultrasonic technology, which stimulates a narrow pulse on one side of the lithium battery and then studies the ultrasonic time domain waveform characteristics of the pulse passing through the lithium battery, such as the intensity of the transmitted wave, to perform non-destructive testing and evaluation of lithium batteries; (2) Ultrasonic guided wave technology, which stimulates ultrasonic guided waves in the battery and uses the scattered waves after the guided waves interact with the defects to detect internal defects of the battery; (3) Ultrasonic emission technology, during the charging and discharging process, the battery will emit ultrasonic signals with inconsistent characteristics. Therefore, by measuring the ultrasonic signals emitted by the battery during the charging and discharging process, it is possible to evaluate the battery SoC; 4) Continuous ultrasonic technology, the existing related research is to place the transmitting and receiving ultrasonic transducers at the center of the two sides of the soft-pack battery, use the sweep frequency signal to stimulate the transmitting transducer to vibrate, and then measure the time domain waveform of the ultrasonic wave after passing through the battery in a certain frequency band under different SoCs, perform Fourier transform on it, and obtain the amplitude-frequency characteristics of the transmitted wave. On this basis, the SoC is evaluated according to the attenuation degree. In general, the ultrasonic non-destructive testing and evaluation technology for lithium-ion batteries is still in the development stage. Pulsed ultrasonic technology uses high-voltage pulses to excite an ultrasonic transducer, and then studies the amplitude differences in the time-domain waveform of narrow ultrasonic pulses after they pass through different locations on a lithium battery to test the battery. However, this technology only utilizes the amplitude information of the transmitted wave, making it difficult to detect internal defects in the battery that have little effect on the amplitude of the transmitted wave, such as lithium plating or wrinkling in small local areas. Acoustic emission technology has not yet entered the practical stage because the correlation between various battery failures and the characteristics of the ultrasonic signals emitted during their charge and discharge processes remains unclear. The difficulty of ultrasonic guided wave technology lies in the fact that ultrasonic guided waves have multi-mode and dispersive characteristics, and the waveforms after scattering by defects are more complex. Therefore, it is very difficult to assess the health status of lithium-ion batteries from the received guided waves. In addition, ultrasonic guided wave technology can only be used to test thinner solid-state electrolyte batteries. Existing continuous ultrasonic technology has the following shortcomings: due to the use of continuous wave excitation, the excitation signal voltage cannot be too high, otherwise it will damage the transmitting ultrasonic transducer. The ultrasonic signal is severely attenuated in thicker hard-shell lithium batteries, resulting in a low signal-to-noise ratio of the signal collected by the receiving transducer. Therefore, if conventional Fourier transform technology is used to process the received signal, the amplitude-frequency distribution changes of the signal caused by internal defects in the battery may be submerged in the noise, and even effective transmitted wave information cannot be extracted.

[0006] Therefore, it is necessary to develop a new ultrasonic detection technology to overcome the above defects.

[0007] Summary of the Invention

[0008] The purpose of the present invention is to overcome the defects of the prior art and provide a device and method for non-destructive testing of lithium batteries based on ultrasonic resonance spectrum. By accurately measuring the ultrasonic resonance spectrum based on transmitted waves or reflected waves at different positions of the lithium battery, several resonant frequencies are identified from the ultrasonic resonance spectrum and their distribution characteristics are analyzed; the passband and bandgap characteristics in the ultrasonic resonance spectrum signal are analyzed; the amplitude information of the transmitted waves or reflected waves at different frequency points is extracted from the ultrasonic resonance spectrum signal, and the amplitude cloud map of the transmitted waves or reflected waves of the lithium battery at several frequency points, as well as the mechanical quality factor of the lithium battery, are obtained. On this basis, not only can failure states of the lithium battery such as poor wetting, wrinkling, and bubbles be detected, but the lithium battery SoC can also be quantitatively evaluated.

[0009] In order to realize non-destructive testing of lithium batteries based on ultrasonic resonance spectrum, it is first necessary to realize accurate measurement of ultrasonic resonance spectrum based on transmitted wave or reflected wave at different positions of lithium batteries. The present invention proposes to use a frequency sweep signal generation module to generate a frequency sweep signal, which, after being amplified by the power amplifier module, becomes the excitation signal of the transmitting ultrasonic transducer or the dual-crystal ultrasonic transducer transmitting unit. The signal received by the receiving transducer or the dual-crystal ultrasonic transducer transmitting unit is pre-amplified and collected, and the input signal of the transmitting ultrasonic transducer or the dual-crystal ultrasonic transducer transmitting unit is used as the reference signal to perform phase-locked processing and amplification to obtain the ultrasonic resonance spectrum based on transmitted wave or reflected wave of the lithium battery at the measurement position. The reason for using phase-locked amplification is that this technology can realize accurate measurement of weak signals. If the ultrasonic resonance spectrum is obtained by Fourier transforming the received time domain signal, the signal-to-noise ratio is low, and many useful signals are submerged in the noise and difficult to extract.

[0010] Secondly, it is necessary to establish the corresponding relationship between the ultrasonic resonance frequency characteristics of healthy lithium batteries and the geometric and material parameters of lithium batteries. Soft-pack lithium batteries can be equivalent to transversely isotropic materials, and the material parameters include density and elastic coefficient c 11 、c 12 、c 13 、c 33 、c 44 The shell of a hard-shell lithium battery is generally made of isotropic materials, such as aluminum alloy, and its material parameters include density and elastic coefficient c 11 with c 12 The inner core of the hard shell lithium battery is immersed in liquid electrolyte, which can also be equivalent to a transversely isotropic material. If we only focus on the propagation of longitudinal waves perpendicular to the isotropic surface, we only consider c 33That’s it. The principle and method for establishing the correspondence between the ultrasonic resonance frequency characteristics of healthy lithium batteries and the geometric and material parameters of lithium batteries are as follows: For hard-shell lithium batteries, the resonance frequency is determined by the thickness of the outer shell and the material parameters, the thickness of the battery core and the equivalent material parameters. For soft-pack batteries, the resonance frequency is determined by its thickness and equivalent material parameters. On the contrary, under the premise that the geometric parameters (thickness) of each part of the battery have been determined, if several resonance frequencies of the lithium battery are measured, its material parameters can be inverted by the nonlinear least squares method. By measuring the ultrasonic resonance spectrum of a sufficient number of lithium batteries and identifying enough resonance frequencies, the material parameters of healthy lithium batteries can be reasonably characterized, and at the same time, several resonance frequencies of healthy lithium batteries can be determined. After determining the material parameters and ultrasonic resonance spectrum characteristics (such as resonance frequency) of healthy lithium batteries, use this as a reference standard to measure the ultrasonic resonance spectrum at different positions of the lithium battery, and perform characteristic analysis to make a preliminary judgment on whether the lithium battery has abnormalities.

[0011] Furthermore, it is necessary to establish the influence of the coating of the inner core pole piece and the wrinkling of the diaphragm of the lithium battery on the changes in the ultrasonic resonance spectrum characteristics. The finite element method is used to simulate the influence of different degrees of distortion and wrinkling of different sizes on the ultrasonic resonance spectrum characteristics (such as resonant frequency and spectrum amplitude distribution, etc.), and combined with the actual measurement results of the ultrasonic resonance spectrum of lithium batteries with inner core pole piece coating and diaphragm wrinkling, the relationship between the coating of the inner core pole piece and the wrinkling of the diaphragm of the lithium battery and the changes in the ultrasonic resonance spectrum characteristics can be established. On this basis, the lithium battery is scanned point by point, the ultrasonic resonance spectrum at several positions is measured, and the spectrum characteristics are analyzed. It is possible to make quantitative detection of whether there is wrinkling in the inner core of the lithium battery, the degree of wrinkling, and the size of the wrinkling area. The principle of the above invention concept is that the coating of the inner core pole piece and the wrinkling area of ​​the diaphragm of the lithium battery have distortions in geometry and material properties, and the ultrasonic resonance spectrum characteristics of the lithium battery are determined by its geometry and material properties. Therefore, the coating of the pole piece and the wrinkling of the diaphragm will significantly change the ultrasonic resonance spectrum characteristics at this position. By measuring the ultrasonic resonance spectra at healthy locations of lithium batteries and at locations where electrode coating and diaphragm wrinkling exist, and comparing spectral characteristics such as the ultrasonic resonance frequency, it is possible to evaluate whether wrinkling exists and the size of the wrinkled area.

[0012] Furthermore, it is necessary to establish the impact of bubbles inside lithium batteries on the ultrasonic resonance spectrum characteristics (including the resonant frequency distribution and spectral amplitude distribution). Finite element methods were used to simulate the impact of spherical and ellipsoidal bubbles of different sizes on the ultrasonic resonance spectrum of lithium batteries. A simulated test model of a lithium battery containing bubbles was tested to verify the numerical results. Combined with actual measurement results of the ultrasonic resonance spectrum of a lithium battery containing bubbles inside its core, the relationship between the shape and size of bubbles inside the lithium battery and the ultrasonic resonance spectrum characteristics was established. On this basis, the lithium battery was scanned point by point, and the ultrasonic resonance spectrum at several locations was measured. Analyzing the spectral characteristics, particularly the distribution of the ultrasonic resonance frequency, it was possible to determine whether bubbles were present in the lithium battery core and to quantitatively assess the bubble shape and size. The characteristic acoustic impedance of bubbles differs significantly from that of the battery core. Therefore, when ultrasonic waves are incident on the bubble surface, they will produce strong scattering. The ultrasonic waves transmitted and reflected by the lithium battery inevitably carry ultrasonic scattering signals. The received transmitted and reflected ultrasonic waves are phase-locked with the excitation signal of the transmitting transducer to obtain ultrasonic resonance spectra based on the transmitted and reflected waves, respectively. These spectra inevitably contain the resonance signals of the bubbles.

[0013] Furthermore, it is necessary to establish a relationship between the degree of electrolyte wetting in lithium batteries and the amplitude of reflected or transmitted waves. Poorly wetted areas in lithium batteries can be considered as discontinuous regions of the dielectric that are difficult for sound waves to penetrate. Finite element methods were used to simulate the effect of the size of the dielectric discontinuous region within lithium batteries on the ultrasonic resonance spectrum of lithium batteries. Combined with actual measurement results of the ultrasonic resonance spectrum of poorly wet lithium batteries and disassembly verification, a relationship between poor wetting and the characteristics of the ultrasonic resonance spectrum of lithium batteries was established. On this basis, the lithium battery was scanned point by point, and the ultrasonic resonance spectra based on reflected or transmitted waves were measured at several locations. The spatial distribution of the spectrum amplitude at several frequency points was extracted from this data. This allowed a determination of whether the lithium battery core had poor wetting and a quantitative assessment of the size of the poorly wetted area. Lithium battery cores with well-wetted electrolytes can be considered as solid-liquid two-phase media with good sound transmission and weak internal reflection signals. However, poorly wetted electrolyte areas have poor transmission, or even impervious to sound waves, resulting in relatively strong reflection signals. Furthermore, poorly wetted areas within the battery can alter the resonant frequency distribution characteristics of the battery. Therefore, by analyzing the resonant frequency distribution characteristics in the ultrasonic resonance spectrum based on the reflected wave or transmitted wave, as well as the reflected wave or transmitted wave amplitude at several frequency points, the poor electrolyte infiltration of the lithium battery can be evaluated.

[0014] Furthermore, it is necessary to establish the amplitude of the specific frequency wave in the ultrasonic resonance spectrum based on the transmitted wave, as well as the mechanical quality factor (Q M ) is quantitatively related to SoC. The sharpness of the resonance peak reflects the Q of the lithium battery. M , the sharper the resonance peak, the higher the Q M The higher the value, the flatter the resonance peak, indicating that the Q MBy measuring the ultrasonic resonance spectrum of healthy lithium batteries at different SoCs and analyzing it, we can obtain the quantitative relationship between the amplitude of the specific frequency wave in the ultrasonic resonance spectrum and the SoC, as well as the Q M On this basis, the ultrasonic resonance spectrum of the lithium battery based on the transmitted wave is measured, and the spectrum amplitude at several frequencies and the Q corresponding to several resonance peaks are extracted. M , the SoC of lithium batteries can be quantitatively evaluated. The acoustic properties of the lithium battery core change with the change of SoC, that is, under different SoCs, the lithium battery core has different acoustic characteristic impedance and damping. The amplitude of the ultrasonic resonance spectrum based on the transmission wave reflects the amplitude of ultrasonic waves of different frequencies after penetrating the lithium battery. Therefore, by measuring the ultrasonic resonance spectrum under different SoCs, the relative amplitude of ultrasonic waves of any frequency after penetrating the lithium battery within the frequency band covered by the ultrasonic resonance spectrum under different SoCs can be obtained. In addition, the physical quantity corresponding to the sharpness of each resonance peak, that is, Q M It reflects the magnitude of the material damping. Therefore, according to the ultrasonic resonance spectrum of the lithium battery measured under different SoC, the relationship between the lithium battery SoC and Q can also be obtained. M On the contrary, under the same test conditions, from the measured lithium battery ultrasonic resonance spectrum, extract the relative amplitude of several different frequency ultrasonic waves after penetrating the lithium battery, and extract the Q corresponding to several resonance peaks. M , we can know the SoC of lithium battery.

[0015] In order to achieve the above objectives, one of the technical solutions of the present invention is: a device for non-destructive testing of lithium batteries based on ultrasonic resonance spectrum, including a sweep signal generation module, a power amplification module, a transmitting ultrasonic transducer, a receiving ultrasonic transducer, a preamplifier module, a data acquisition module, and a phase-locked amplification module.

[0016] Furthermore, the transmitting ultrasonic transducer and the receiving ultrasonic transducer obtain ultrasonic resonance spectra based on transmitted waves, and the transmitting ultrasonic transducer and the receiving ultrasonic transducer are respectively placed on both side surfaces of the lithium battery through a coupling agent.

[0017] Furthermore, the transmitting ultrasonic transducer and the receiving ultrasonic transducer are placed on the same side surface of the lithium battery via a coupling agent, or are replaced with a dual-crystal ultrasonic transducer with integrated transmitting and receiving functions to obtain an ultrasonic resonance spectrum based on the reflected wave. The dual-crystal ultrasonic transducer includes a transmitting unit and a receiving unit, and the dual-crystal ultrasonic transducer is placed on the side surface of the lithium battery via a coupling agent. The function of the transmitting unit is to excite a broadband ultrasonic signal, and the function of the receiving unit is to receive the ultrasonic signal reflected from the lithium battery.

[0018] The function of the transmitting ultrasonic transducer is to excite a broadband ultrasonic signal, and the function of the receiving ultrasonic transducer is to receive the ultrasonic signal that penetrates the lithium battery. The function of the sweep frequency signal generating module is to generate a broadband sweep frequency signal. The function of the power amplifier module is to amplify the sweep frequency signal generated by the sweep frequency signal generating module, and the amplified signal serves as the excitation signal for the transmitting unit in the dual-crystal ultrasonic transducer. The function of the preamplifier module is to amplify the output signal of the receiving unit. The function of the data acquisition module is to perform analog-to-digital conversion and acquisition on the output signal of the preamplifier module. The function of the phase-locked amplifier module is to use the sweep frequency signal generated by the sweep frequency signal generating module as a reference signal, perform phase-locked amplification on the collected reflected wave signal, and generate an ultrasonic resonance spectrum.

[0019] In order to achieve the above objectives, the second technical solution of the present invention is: the application of a device for non-destructive testing of lithium batteries based on ultrasonic resonance spectroscopy in detecting the failure state of lithium batteries and quantitatively evaluating the SoC of lithium batteries.

[0020] To achieve the above objectives, the third technical solution of the present invention is: a method for detecting the failure state of a lithium battery using an ultrasonic resonance spectrum non-destructive testing device for lithium batteries, which specifically includes the following steps:

[0021] (1) The swept frequency signal is amplified as an excitation signal for transmitting ultrasound, and the swept frequency signal is used as a reference signal to perform phase-locking processing on the received ultrasound signal to obtain the ultrasonic resonance spectrum based on the transmitted wave or reflected wave at different positions of the lithium battery;

[0022] (2) Identify several resonant frequencies from the ultrasonic resonance spectrum and analyze the characteristics of the ultrasonic resonance spectrum;

[0023] (3) Analyze the passband and forbidden band characteristics in the ultrasonic resonance spectrum signal;

[0024] (4) Extract the amplitude information of the transmitted or reflected waves at different frequency points from the ultrasonic resonance spectrum signal, and obtain the amplitude cloud map of the lithium battery transmitted or reflected waves at several frequency points and the mechanical quality factor Q of the lithium battery M =f / Δf, where f is the resonant frequency and Δf is the -3dB bandwidth;

[0025] (5) Detect the failure status of lithium batteries.

[0026] Furthermore, in step (1), the ultrasonic wave is transmitted by a transmitting ultrasonic transducer or a dual-crystal transducer transmitting unit, and the ultrasonic wave is received by a receiving ultrasonic transducer or a dual-crystal transducer receiving unit. When the transmitting and receiving ultrasonic transducers are placed on opposite sides of the battery, an ultrasonic resonance spectrum based on the transmitted wave can be obtained; when the transmitting and receiving ultrasonic transducers are placed on the same side of the battery, or a dual-crystal ultrasonic transducer is used, an ultrasonic resonance spectrum based on the reflected wave can be obtained.

[0027] Furthermore, the resonant frequency in step (2) refers to the frequency corresponding to each resonant peak in the ultrasonic resonance spectrum. Theoretically, there are infinite resonant frequencies of lithium batteries, but in actual measurement, only the resonant frequencies within the scanning frequency band can be measured.

[0028] Furthermore, the ultrasonic resonance spectrum characteristics in step (2) mainly include the magnitude and distribution of the resonance frequency, the spectrum amplitude distribution and the sharpness of the resonance peak.

[0029] Furthermore, the bandwidth of the ultrasonic resonance spectrum in steps (1)-(4) is selected to be 10kHz-10MHz.

[0030] Furthermore, in step (5), the failure state of the lithium battery can be determined based on the ultrasonic resonance spectrum of the transmitted wave or the reflected wave, and the quantitative evaluation of the lithium battery SoC is performed based on the ultrasonic resonance spectrum of the transmitted wave.

[0031] Furthermore, the method for detecting poor wetting in the failure state of the lithium battery in step (5) comprises the following steps:

[0032] (1) Analyze the changes in the resonant frequency value and the number of resonant modes in the ultrasonic resonance spectrum based on reflected waves or transmitted waves at different spatial positions of the lithium battery, as well as the changes in the sharpness of the resonance peak;

[0033] (2) Based on the ultrasonic resonance spectrum of the reflected wave, the signal amplitude of the reflected wave at several frequency points at all scanning measurement points can be extracted, and further the spatial distribution cloud map of the reflected wave amplitude at several frequency points can be obtained;

[0034] (3) Based on the ultrasonic resonance spectrum of the transmitted wave, the signal amplitude of the transmitted wave at several frequency points at all scanning measurement points can be extracted, and further the spatial distribution cloud map of the transmitted wave amplitude at several frequency points can be obtained;

[0035] (4) Based on the above spatial distribution cloud map, the areas where the lithium battery has poor wetting are visually identified.

[0036] The inner core of a lithium battery with good infiltration effect can be regarded as a solid-liquid two-phase medium with good acoustic transmission performance and weak internal reflection signal. However, in the area where the lithium battery electrolyte is poorly infiltrated, the ultrasonic transmission performance is significantly reduced, and the amplitude of the transmitted wave is often significantly lower than that of the healthy position, with the difference even reaching 100 times. However, the amplitude of the reflected wave is significantly increased. Therefore, based on the spatial distribution cloud map of the reflected wave or transmitted wave amplitude at several frequency points, the area of ​​the lithium battery with poor infiltration can be intuitively identified, thereby evaluating the poor electrolyte infiltration of the lithium battery.

[0037] Scanning the poorly wetted areas of lithium battery electrolytes once with a transmitted wave and then once with a reflected wave, and then using the results of both scans for a comprehensive assessment, is more reliable than using only the transmitted wave signal to detect poor wetting.

[0038] The geometric and material properties of the lithium battery core are distorted in the areas where the pole piece coating and diaphragm wrinkle. The ultrasonic resonance spectrum characteristics of lithium batteries are determined by their geometry and material properties. Therefore, pole piece coating and diaphragm wrinkling will significantly change the ultrasonic resonance spectrum characteristics at this location. The swept frequency signal is power-amplified and used as the excitation signal for the transmitting ultrasonic transducer or the transmitting unit of the dual-crystal ultrasonic transducer. This swept frequency signal is used as the reference signal to phase-lock the reflected wave or transmitted wave signal received by the receiving ultrasonic transducer or the receiving unit of the dual-crystal ultrasonic transducer to obtain the lithium battery ultrasonic resonance spectrum based on the transmitted wave or reflected wave. By measuring the ultrasonic resonance spectra at healthy locations of the lithium battery and at locations with pole piece coating and diaphragm wrinkling, and analyzing and comparing the ultrasonic resonance spectrum characteristics based on the transmitted wave or reflected wave, it is possible to evaluate the presence of wrinkles in the pole piece coating and diaphragm inside the lithium battery and the size of the wrinkled area.

[0039] The ultrasonic resonance spectrum characteristics of lithium batteries are determined by the geometric and material properties of lithium batteries. The wrinkling of the inner core not only changes the geometric characteristics of the battery, but may even change the material properties of the battery, thereby causing changes in the ultrasonic resonance spectrum characteristics.

[0040] Furthermore, the method for detecting wrinkles in a lithium battery failure state in step (5) comprises the following steps:

[0041] (1) Determine the ultrasonic resonance spectrum characteristics of healthy lithium batteries through actual measurement and finite element simulation;

[0042] (2) Determine the ultrasonic resonance spectrum characteristics obtained by measuring the ultrasonic wave penetrating the electrode coating and diaphragm wrinkling position of the lithium battery core or being reflected by the wrinkling position through actual measurement, finite element simulation, battery disassembly verification, etc.

[0043] (3) Establish a library of ultrasonic resonance spectrum characteristics of healthy and wrinkled lithium batteries. Based on this, analyze the ultrasonic resonance spectrum characteristics measured at different positions of the lithium battery to determine whether there is electrode coating and diaphragm wrinkling in the lithium battery core and its distribution range.

[0044] After power amplification, the swept frequency signal is used as the excitation signal of the transmitting ultrasonic transducer or the transmitting unit of the dual-crystal ultrasonic transducer, and the swept frequency signal is used as the reference signal to perform phase-locking processing on the reflected wave or transmitted wave signal received by the receiving ultrasonic transducer or the receiving unit of the dual-crystal ultrasonic transducer to obtain the ultrasonic resonance spectrum of the lithium battery based on the transmitted wave or the reflected wave. By analyzing the ultrasonic resonance spectrum characteristics based on the reflected wave and the transmitted wave, the presence of bubbles inside the lithium battery and the size of the bubbles can be evaluated.

[0045] Furthermore, the method for detecting bubbles in the failed state of the lithium battery in step (5) includes the following steps:

[0046] (1) Determine the ultrasonic resonance spectrum characteristics of healthy lithium batteries through actual measurement and finite element simulation;

[0047] (2) Using the finite element method to simulate the ultrasonic resonance spectrum characteristics after ultrasound is scattered by spherical and ellipsoidal bubbles of different sizes inside the lithium battery;

[0048] (3) Testing lithium batteries that have been confirmed to have bubbles in the core using other detection technologies and analyzing the ultrasonic resonance spectrum characteristics;

[0049] (4) Based on the results of actual measurements and numerical simulations, the qualitative and quantitative effects of the shape and size of bubbles inside lithium batteries on the characteristics of the ultrasonic resonance spectrum are clarified. Based on this, the ultrasonic resonance spectrum characteristics measured at different positions of the lithium battery are analyzed to determine whether there are bubbles inside the lithium battery core and the size of the bubbles.

[0050] The acoustic properties of the lithium battery core change with the change of SoC, that is, under different SoCs, the lithium battery core has different acoustic characteristic impedance and damping. The amplitude of the ultrasonic resonance spectrum based on the transmission wave reflects the amplitude of ultrasonic waves of different frequencies after penetrating the lithium battery. Therefore, by measuring the ultrasonic resonance spectrum under different SoCs, the relative amplitude of ultrasonic waves of any frequency after penetrating the lithium battery within the frequency band covered by the ultrasonic resonance spectrum under different SoCs can be obtained. In addition, the physical quantity corresponding to the sharpness of each resonance peak, that is, the mechanical quality factor (Q M ) reflects the magnitude of the material damping. Therefore, according to the ultrasonic resonance spectrum of the lithium battery measured under different SoC, the relationship between the lithium battery SoC and Q can also be obtained. M On the contrary, under the same test conditions, from the measured lithium battery ultrasonic resonance spectrum, extract the relative amplitude of several different frequency ultrasonic waves after penetrating the lithium battery, and extract the Q corresponding to several resonance peaks. M , we can know the SoC of lithium battery.

[0051] To achieve the above objectives, the fourth technical solution of the present invention is: a method for quantitatively evaluating the SoC of a lithium battery based on an ultrasonic resonance spectroscopy non-destructive testing device for lithium batteries, specifically comprising the following steps:

[0052] (1) Measure the ultrasonic resonance spectrum of healthy lithium batteries based on transmitted waves at different SoCs, extract the signal amplitudes at several frequency points, and establish the corresponding relationship between SoC and the signal amplitudes at several frequencies;

[0053] (2) Select several resonance peaks in the ultrasonic resonance spectrum and calculate the value according to the formula Q M =f max / Δf, where Δf is the 3dB bandwidth, calculate the Q corresponding to each peak M value, establish SoC with each Q M The correspondence of values;

[0054] (3) Scan and measure the ultrasonic resonance spectrum based on the transmitted wave at different spatial positions of the lithium battery to be tested, and extract the signal amplitude at several frequency points and the Q corresponding to several resonance peaks. M Value, combined with SoC and signal amplitude at different frequencies, and SoC and Q M The relationship between the values ​​is used to evaluate the SoC of the lithium battery to be tested.

[0055] Compared with the prior art, the present invention has the following beneficial effects:

[0056] 1. The ultrasonic resonance spectrum used in the present invention is rich in information. For healthy lithium batteries, the ultrasonic resonance spectrum includes resonance information of the battery shell and inner core. For defective lithium batteries, it also includes resonance information of defects such as poor infiltration, wrinkling, and bubbles in the lithium battery inner core. In addition, the ultrasonic resonance spectrum based on the transmitted wave also includes attenuation information of ultrasonic waves of different frequencies after penetrating the lithium battery. The ultrasonic resonance spectrum based on the reflected wave also includes amplitude information of ultrasonic waves of different frequencies after being reflected by internal defects in the lithium battery. Therefore, compared with existing ultrasonic technology, ultrasonic resonance spectroscopy technology can implement more comprehensive non-destructive testing and evaluation of lithium batteries;

[0057] 2. The present invention utilizes phase-locked amplification technology, which excels in weak signal detection. Compared to techniques that obtain a signal spectrum by Fourier transforming the time-domain waveform of a swept-frequency ultrasonic signal after it penetrates a lithium battery, this technology enables more accurate measurement of the ultrasonic resonance spectrum of a lithium battery, preventing the loss or drowning of weak information due to the interaction between internal battery defects and the ultrasonic wave. When high-frequency continuous ultrasonic waves are used for lithium battery testing, the excitation power should not be too high, as this can easily overheat the ultrasonic transducer and damage it. However, when testing larger hard-shell lithium batteries, insufficient excitation signal strength can further attenuate the ultrasonic wave as it propagates through the battery core, resulting in a very weak transmitted wave signal.

[0058] 3. Both the reflected wave and transmitted wave ultrasonic resonance spectra of the present invention can accurately detect defects such as poor wetting, wrinkles, and bubbles in the lithium battery core;

[0059] 4. The ultrasonic resonance spectroscopy technique of the present invention can reliably detect bubbles. However, using pulsed ultrasonic transmission wave amplitude maximum value changes in the time domain waveform is difficult to detect tiny bubbles in hard-shell liquid electrolyte lithium batteries because it is insensitive to the maximum amplitude changes in the time domain waveform of the transmission wave. However, the resonance information of bubbles will be reflected in the ultrasonic resonance spectrum based on the reflected wave or the transmitted wave. Therefore, the ultrasonic resonance spectroscopy technique of the present invention can reliably detect them.

[0060] 5. The ultrasonic resonance spectrum technology of the present invention can provide a more reliable quantitative assessment of SoC. Compared with the SoC detection based on the change of the maximum amplitude of the pulsed ultrasonic transmission wave in the time domain, the ultrasonic resonance spectrum technology can not only accurately obtain the amplitude attenuation of several single-frequency ultrasonic waves after penetrating the lithium battery, but also extract the Q value from the resonance peak. M Value; waves of different frequencies propagate at different attenuations in the same medium, Q M The value directly reflects the magnitude of the material damping, which is based on the amplitude attenuation information of multiple frequency ultrasonic waves after penetrating the lithium battery, and combined with the Q M The present invention can perform a more reliable quantitative evaluation of SoC. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 is a structural block diagram of a lithium battery ultrasonic resonance spectrum test system based on transmitted waves;

[0062] FIG2 is an ultrasonic resonance spectrum obtained by measuring a healthy part and a wrinkled part of the inner core of a hard-shell lithium battery in Example 2;

[0063] FIG3 is an ultrasonic resonance spectrum of a healthy part and a severely poorly infiltrated part of a hard-shell lithium battery in Example 3;

[0064] FIG4 is an ultrasonic resonance spectrum of a healthy part and a poorly infiltrated part of a hard-shell lithium battery in Example 3;

[0065] FIG5 is a schematic diagram of the grid division of the hard shell lithium battery surface test in Example 3;

[0066] FIG6 is a spatial cloud diagram of the amplitude distribution of the transmitted wave obtained by extracting the spectrum amplitude information at several frequency points from the ultrasonic resonance spectrum of the poorly infiltrated lithium battery in Example 3;

[0067] FIG. 7 shows the poor wetting area of ​​the lithium battery in Example 3. DETAILED DESCRIPTION

[0068] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention is described in more detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited to these embodiments. The same reference numerals throughout the text represent the same elements, and similar reference numerals represent similar elements.

[0069] In the description of the present invention, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "horizontal", "vertical", "top", "bottom", "inside" and "outside" and the like to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the stereoscopic diagrams in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0070] Example 1

[0071] A device for nondestructive testing of lithium batteries based on ultrasonic resonance spectrum, as shown in Figure 1, is a lithium battery ultrasonic resonance spectrum test system structure based on transmitted waves, including a sweep signal generation module, a power amplifier module, a preamplifier module, a data acquisition module, a phase-locked amplifier module, a transmitting ultrasonic transducer and a receiving ultrasonic transducer.

[0072] The transmitting ultrasonic transducer and the receiving ultrasonic transducer are attached to the surfaces of both sides of the lithium battery to be tested through ultrasonic coupling agent. In order to obtain better consistency, a water immersion ultrasonic transducer can also be selected, and the lithium battery can be placed in water or silicone oil for water immersion or oil immersion measurement.

[0073] The ultrasonic resonance spectrum test process based on transmitted waves is as follows: the upper computer software on the industrial computer controls the sweep frequency signal generation module to generate a sweep frequency signal. After being amplified by the power amplifier module, the sweep frequency signal becomes the excitation signal of the transmitting ultrasonic transducer. The transmitting ultrasonic transducer excites ultrasonic waves on the surface of the lithium battery. After the ultrasonic waves penetrate the battery, they are received by the receiving ultrasonic transducer. The signal received by the receiving ultrasonic transducer is first amplified by the preamplifier module, and then collected by the data acquisition module and input into the phase-locked amplifier module. The sweep frequency signal generated by the sweep frequency signal generation module is also input into the phase-locked amplifier module as a reference signal for phase-locked amplification. The phase-locked amplifier module performs phase-locking processing on the two to obtain the lithium battery ultrasonic resonance spectrum based on the transmitted wave at the measurement position.

[0074] The center operating frequency of the transmitting and receiving ultrasonic transducers must be determined based on the thickness of the battery being tested and generally must be higher than the highest frequency of the ultrasonic resonance spectrum of the sample being tested. For example, to test a hard-shell lithium-ion battery approximately 7 cm thick, a broadband ultrasonic transducer with a center operating frequency in the 2MHz-5MHz range can be selected. The bandwidth of the swept frequency signal generation module, power amplifier module, preamplifier module, data acquisition module, and phase-locked amplifier module must be higher than the bandwidth of the ultrasonic resonance spectrum to be measured.

[0075] For example, the transmitting and receiving ultrasonic transducers in Figure 1 can be replaced with a dual-crystal ultrasonic transducer that integrates both transmitting and receiving functions. The housing of the dual-crystal ultrasonic transducer houses two independent piezoelectric crystals: one crystal is used to transmit ultrasound, the transmitting unit, and the other is used to receive ultrasound, the receiving unit. The signal output from the receiving unit undergoes preamplification and data acquisition, and then is input into a phase-locked amplifier module and a swept-frequency signal generator module for phase-locked processing. This allows the ultrasonic resonance spectrum of the lithium battery based on the reflected wave to be obtained. The ultrasonic resonance frequency is the frequency corresponding to each resonant peak in the ultrasonic resonance spectrum. Theoretically, lithium batteries have an infinite number of resonant frequencies, but in actual measurements, only resonant frequencies within the swept-frequency band can be measured. The characteristics of the ultrasonic resonance spectrum primarily include the magnitude and distribution of the resonant frequencies, the spectral amplitude distribution, and the sharpness of the resonant peaks. The characteristics of the lithium battery ultrasonic resonance spectrum are determined by the battery's geometry and material properties. Wrinkling the inner core not only changes the battery's geometry but can also alter its material properties, thereby causing changes in the ultrasonic resonance spectrum.

[0076] Example 2

[0077] A device for nondestructive testing of lithium batteries based on ultrasonic resonance spectroscopy is used for detecting electrode coating and diaphragm wrinkling in hard-shell liquid electrolyte lithium batteries.

[0078] The battery under test is a hard-shell liquid electrolyte lithium battery with a length, width, and height of 17.5 cm, 7.0 cm, and 20 cm, respectively. The battery is placed on the lifting platform shown in Figure 1. Adjusting the lifting platform controls the vertical movement of the battery. The three-dimensional movable platform in Figure 1 adjusts the three-dimensional movement of the transmitting and receiving ultrasonic transducers, enabling point-by-point scanning testing of the lithium battery. The center frequency of the transmitting and receiving ultrasonic transducers is 4.0 MHz, with a bandwidth of 85% (a performance parameter of the transducers), and the test bandwidth is 10 kHz to 1000 kHz.

[0079] The starting frequency, cutoff frequency, total scanning time, and duration of the signal at each frequency point of the sweep signal are set through the host software installed on the industrial computer. After the sweep signal is amplified by the power amplifier, the voltage amplitude is 20V. The amplified signal excites the transmitting ultrasonic transducer to produce vibration. Due to the use of continuous wave excitation, the excitation voltage cannot be too high, otherwise the transmitting ultrasonic transducer will be damaged. The signal received by the receiving transducer is pre-amplified and data collected, and then input into the phase-locked amplifier. The sweep signal generated by the sweep signal generation module is used as the reference signal for phase-locked processing to obtain the ultrasonic resonance spectrum of the battery to be tested based on the transmitted wave. Save the data after the test is completed. Control the three-dimensional motion platform to move the transmitting and receiving ultrasonic transducers, repeat the above process for measurement, and thus complete the scanning test of the ultrasonic resonance spectrum at several spatial points of the lithium battery.

[0080] The solid line in Figure 2 represents the ultrasonic resonance spectrum of a healthy lithium battery, while the dashed line represents the ultrasonic resonance spectrum of a wrinkled core defect. Extensive testing has shown that healthy lithium batteries of the same type exhibit essentially identical ultrasonic resonance spectrum characteristics. The battery used in the test in Figure 2 was disassembled to identify both healthy and wrinkled parts of the battery core. Each peak in Figure 2 corresponds to a resonant frequency. Figure 2 shows that between 80kHz and 250kHz, the number and location of ultrasonic resonance peaks significantly change due to wrinkling in the lithium battery core. Furthermore, between 230kHz and 250kHz, wrinkling in the battery core significantly reduces the amplitude of the transmitted wave signal. However, between 400kHz and 500kHz, wrinkling in the battery core actually increases the amplitude of the transmitted wave signal. Based on these ultrasonic resonance spectrum characteristics, the presence of wrinkling in the battery core can be quickly determined.

[0081] Example 3

[0082] A device for nondestructive testing of lithium batteries based on ultrasonic resonance spectrum is used to detect poor wetting of the inner core of a hard-shell liquid electrolyte lithium battery.

[0083] Using the test setup shown in Figure 1, a hard-shell liquid electrolyte lithium battery, which had been confirmed to have poor wetting by other testing techniques, was tested. A well-wetted lithium battery core can be considered a solid-liquid two-phase medium with good acoustic transparency. Poorly wetted areas of the lithium battery can be considered discontinuous regions of the medium that are difficult for sound waves to penetrate.

[0084] The solid line in Figure 3 represents the ultrasonic resonance spectrum of a healthy lithium battery cell, while the dashed line represents the ultrasonic resonance spectrum of a core cell with severe poor wetting defects. Figure 3 shows that within the ultrasonic passband of healthy lithium battery cells, poor wetting defects render ultrasonic waves virtually impermeable. Within the 90kHz-500kHz frequency band, the amplitude of the ultrasonic transmission wave in healthy cells is 70 times higher than that in poorly wetting areas. In certain frequency bands, such as around 280kHz, the amplitude is even 100 times higher.

[0085] In Figure 4, line 1 is the ultrasonic resonance spectrum of a healthy part of the lithium battery, and line 2 is the ultrasonic resonance spectrum of a part of the inner core with poor infiltration defects. Figure 4 shows that in the poorly wetted area, considering the transmitted wave within the bandwidth of 94.1kHz-352.3kHz, not only the amplitude is significantly reduced, but also the resonant frequency value changes within this frequency band, and the number of resonant modes decreases, and the sharpness of the resonant peak decreases. For example, in the frequency band of 90-181kHz, 9 resonant peaks can be observed in curve 1, and the resonant frequencies are 94.575kHz, 104.214kHz, 114.319kHz, 124.580kHz, 134.530kHz, 146.501kHz, 156.762kHz, 167.334kHz and 176.196kHz respectively; while only 7 resonant peaks can be observed in curve 2, and the resonant frequencies are 97.839kHz, 108.545kHz, 126.290kHz, 134.219kHz, 141.526kHz, 168.267kHz and 176.963kHz respectively.

[0086] In Figure 5, line 1 is the ultrasonic resonance spectrum of the healthy part of the lithium battery, and line 2 is the ultrasonic resonance spectrum of the part with poor infiltration defects in the inner core. The bandwidth is 800-1100kHz, and each peak in the figure corresponds to a resonance mode. According to the calculation formula of the mechanical quality factor QM, the Q corresponding to the resonance frequency f1 of the healthy position and the poor infiltration of the lithium battery can be calculated. M They are 59 and 102 respectively, corresponding to the Q of the resonant frequency f2 at the healthy position and poorly infiltrated position of the lithium battery. M are 70 and 114 respectively, corresponding to the Q of the resonant frequency f3 at the healthy position and poorly infiltrated position of the lithium battery. M 86 and 123 respectively. Q of the lithium battery at the poorly wetted position M The value is significantly higher than the Q at the healthy position M value.

[0087] The surface of the lithium battery is divided into a grid, and then the ultrasonic resonance spectrum of the lithium battery is scanned and measured at the grid points. The grid shown in Figure 6 has a total of 45 grid points. By increasing the grid density, the spatial resolution of the detection results can be improved. From the ultrasonic resonance spectrum measured at each grid point, the spectrum amplitude value at the same frequency is extracted to obtain a cloud map of the spatial distribution of the transmission wave amplitude. Figure 5 is a cloud map of the spatial distribution of the ultrasonic transmission wave amplitude at six frequency points, namely 400kHz, 500kHz, 600kHz, 700kHz, 800kHz, and 900kHz. The darker the color, the lower the amplitude. Figure 7 makes it very easy to identify areas with poor wetting of the lithium battery. In theory, the transmission wave amplitude at each frequency point within the sweep bandwidth can be extracted simultaneously from the ultrasonic resonance spectrum, so that the spatial distribution cloud map of the transmission wave amplitude at each frequency point within the sweep bandwidth can be obtained at the same time, thereby improving the detection reliability.

[0088] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. Industrial Applicability

[0089] The present invention belongs to the technical field of lithium battery detection, and specifically discloses a device and method for non-destructive detection of lithium batteries based on ultrasonic resonance spectrum. The device includes a sweep signal generating module, a power amplification module, a transmitting ultrasonic transducer, a receiving ultrasonic transducer, a preamplifier module, a data acquisition module, and a phase-locked amplification module; the detection method includes power amplifying the sweep signal as an excitation signal of the transmitting transducer, and performing phase-locked processing on the signal received by the receiving transducer to obtain an ultrasonic resonance spectrum based on a transmitted wave and a reflected wave at different positions of the lithium battery; identifying several resonant frequencies from the ultrasonic resonance spectrum and analyzing the ultrasonic resonance spectrum characteristics and the passband and bandgap characteristics in the signal; extracting the amplitude information of the transmitted wave at different frequency points from the ultrasonic resonance spectrum signal, and obtaining a cloud map of the lithium battery transmitted wave amplitude at several frequency points and the mechanical quality factor of the lithium battery; detecting the failure state of the lithium battery and quantitatively evaluating the lithium battery SoC, which has industrial practicality.

Claims

1. A device for nondestructive testing of lithium batteries based on ultrasonic resonance spectroscopy, characterized in that: The invention comprises a sweeping signal generating module, a power amplifying module, a transmitting ultrasonic transducer, a receiving ultrasonic transducer, a preamplifying module, a data acquisition module and a phase-locked amplifying module; the sweeping signal generating module is used to generate a sweeping signal, which becomes an excitation signal of the transmitting ultrasonic transducer after being amplified by the power amplifying module; the signal received by the receiving transducer is preamplified and acquired, and then the input signal of the transmitting ultrasonic transducer is used as a reference signal to perform phase-locked processing and amplification, so as to obtain an ultrasonic resonance spectrum of the lithium battery at the measuring position based on the transmitted wave or the reflected wave.

2. The device according to claim 1, characterized in that The transmitting ultrasonic transducer and the receiving ultrasonic transducer are respectively placed on the two side surfaces of the lithium battery through a coupling agent to obtain an ultrasonic resonance spectrum based on a transmission wave.

3. The device according to claim 1, characterized in that The transmitting ultrasonic transducer and the receiving ultrasonic transducer are placed on the same side of the lithium battery, or a dual-crystal ultrasonic transducer is used to obtain an ultrasonic resonance spectrum based on the reflected wave, and the dual-crystal ultrasonic transducer includes a transmitting unit and a receiving unit.

4. An application of the device as described in any one of claims 1 to 3 in detecting the failure state of a lithium battery and quantitatively evaluating the SoC of a lithium battery.

5. A method for quantitatively evaluating the SoC of a lithium battery using the device according to any one of claims 1 to 2, characterized in that: The steps include: (1) Measure the ultrasonic resonance spectrum of healthy lithium batteries based on transmitted waves at different SoCs, extract the signal amplitudes at several frequency points, and establish the corresponding relationship between SoC and the signal amplitudes at several frequencies; (2) Select several resonance peaks in the ultrasonic resonance spectrum and calculate the value according to the formula Q M =f / Δf, where f is the resonant frequency and Δf is the -3dB bandwidth. Calculate the mechanical quality factor Q corresponding to each peak. M values, establish the SoC and each Q M The correspondence of values; (3) Scan and measure the ultrasonic resonance spectrum based on the transmitted wave at different spatial positions of the lithium battery to be tested, extract the signal amplitude at several frequency points, and the Q corresponding to several resonance peaks M value, combining SoC with signal amplitude at different frequencies, and SoC with Q M The relationship between the values ​​is used to evaluate the SoC of the lithium battery to be tested.

6. A method for detecting the failure state of a lithium battery using the device according to any one of claims 1 to 3, characterized in that: The steps include: (1) The transmitting ultrasonic transducer and the receiving ultrasonic transducer are placed on both sides of the lithium battery or on the same side surface through a coupling agent, and the sweep frequency signal is amplified as an excitation signal for transmitting ultrasound. The frequency sweep signal is used as a reference signal to perform phase-locking processing on the received ultrasonic signal to obtain ultrasonic resonance spectra based on transmitted waves or reflected waves at different positions of the lithium battery; (2) identifying several resonant frequencies from the ultrasonic resonance spectrum and analyzing the characteristics of the ultrasonic resonance spectrum; (3) Analyze the passband and forbidden band characteristics in the ultrasonic resonance spectrum signal; (4) Extract the amplitude information of the transmitted or reflected waves at different frequency points from the ultrasonic resonance spectrum signal, and obtain the amplitude cloud map of the lithium battery transmitted or reflected waves at several frequency points and the mechanical quality factor Q of the lithium battery M =f / Δf, where f is the resonant frequency and Δf is the -3dB bandwidth; (5) Detect the failure status of lithium batteries.

7. The method according to claim 6, characterized in that In the step (1), the ultrasonic transmitter is a transmitting ultrasonic transducer or a dual-crystal transducer transmitting unit, and the ultrasonic receiver is a receiving ultrasonic transducer or a dual-crystal transducer receiving unit.

8. The method according to claim 6, characterized in that The bandwidth of the ultrasonic resonance spectrum in steps (1)-(4) is selected to be 10kHz-10MHz.

9. The method according to claim 6, characterized in that The resonant frequency in step (2) is the frequency corresponding to each resonant peak in the ultrasonic resonance spectrum. The characteristics of the ultrasonic resonance spectrum include the magnitude and distribution of the resonant frequency, the spectrum amplitude distribution and the sharpness of the resonant peak.

10. The method according to claim 6, characterized in that The method for detecting poor wetting in the failure state of the lithium battery in step (5) comprises the following steps: (1) Analyze the changes in the resonant frequency value and the number of resonant modes in the ultrasonic resonance spectrum based on reflected waves or transmitted waves at different spatial positions of the lithium battery, as well as the changes in the sharpness of the resonance peak; (2) extracting the signal amplitude of the reflected wave at several frequency points at all scanning measurement space points based on the ultrasonic resonance spectrum of the reflected wave, and further obtaining a spatial distribution cloud map of the reflected wave amplitude at several frequency points; (3) extracting the signal amplitude of the transmitted wave at a number of frequency points at all scanning measurement space points based on the ultrasonic resonance spectrum of the transmitted wave, and further obtaining a spatial distribution cloud map of the transmitted wave amplitude at a number of frequency points; (4) Use the above information to identify areas of the lithium battery where poor wetting exists.

11. The method according to claim 6, characterized in that The method for detecting wrinkles in the failure state of the lithium battery in step (5) comprises the following steps: (1) Determine the ultrasonic resonance spectrum characteristics of healthy lithium batteries through actual measurement and finite element simulation; (2) Determine the penetration rate of ultrasound into lithium batteries through actual measurement, finite element simulation, and battery disassembly verification. Ultrasonic resonance spectrum characteristics of the battery core electrode coating and the diaphragm wrinkling position or after being reflected by the wrinkled part; (3) Establish a library of ultrasonic resonance spectrum characteristics of healthy and wrinkled lithium batteries. Based on this, analyze the ultrasonic resonance spectrum characteristics measured at different positions of the lithium battery to determine whether there is electrode coating and diaphragm wrinkling in the lithium battery core and its distribution range.

12. The method according to claim 6, characterized in that The method for detecting bubbles in the lithium battery failure state in step (5) comprises the following steps: (1) Determine the ultrasonic resonance spectrum characteristics of healthy lithium batteries through actual measurement and finite element simulation; (2) The finite element method is used to simulate the ultrasonic resonance spectrum characteristics after ultrasound is scattered by spherical and ellipsoidal bubbles of different sizes inside the lithium battery; (3) Testing lithium batteries with confirmed bubbles in the inner core and analyzing the ultrasonic resonance spectrum characteristics; (4) Combining the actual measurement and numerical simulation results, the qualitative and quantitative effects of the shape and size of bubbles inside the lithium battery on the ultrasonic resonance spectrum characteristics are clarified, and based on this, the ultrasonic resonance spectrum characteristics measured at different positions of the lithium battery are analyzed. Determine whether there are bubbles in the lithium battery core and the size of the bubbles.

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