Method and device for detecting magnetic impurities in graphite powder

Abstract

The invention provides a graphite powder magnetic impurity detection method and device, and the method comprises the following steps: placing a graphite powder sample in a non-magnetic container, and then applying a vertical vibration force to the sample to make different substances in the sample generate relative motion; the vibration response characteristics of different substances are captured through a magnetic sensor, and then the impurity signal difference is analyzed. Vibration excitation in the vertical direction is innovatively adopted, and the graphite powder generates loosening and relative movement through the superposition effect of the gravity direction and vibration inertia; the magnetic impurities and the graphite particles generate different vibration responses (different phase differences, amplitudes and harmonic components) under the action of inertia difference; the difference causes local magnetic field disturbance and becomes a distinguishable signal source for subsequent magnetic detection.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion electrode material quality detection, in particular to a graphite powder magnetic impurity detection method and device. BACKGROUND

[0002] Lithium ion batteries are widely used in new energy vehicles, energy storage and consumer electronics, etc., wherein the purity and quality of the negative electrode material graphite powder directly determine the safety and service life of the battery.

[0003] During the preparation and processing of graphite powder, magnetic metal impurities such as iron filings, nickel filings and cobalt filings may be mixed in. These impurities are prone to form local current concentration after the battery is formed, inducing micro-short circuit and local overheating, and even leading to battery thermal runaway. Therefore, detection and control of magnetic impurities in graphite powder is an important link in the battery industry chain.

[0004] Currently, the detection of magnetic impurities in graphite powder at the ppb and ppm level mainly relies on the following methods: Microscope observation / chemical analysis: requires sampling and sample preparation, low detection efficiency, and difficult to be applied on a large scale; ICP-MS (inductively coupled plasma mass spectrometry): high sensitivity, but high testing cost and long time-consuming, not suitable for rapid detection; Magnetic separation method: can remove part of the ferromagnetic impurities, but cannot quantitatively detect the impurities; X-ray or CT imaging: can detect internal defects, but the equipment is expensive and the resolution is limited.

[0005] These methods generally have the problems of low efficiency, high cost and difficulty in realizing real-time or online detection, and cannot effectively distinguish the dynamic response of the graphite main body and the trace magnetic impurities. SUMMARY

[0006] The technical problem to be solved by the application is how to detect magnetic impurities in graphite powder at low cost and high efficiency.

[0007] The application solves the above technical problems by the following technical means: The graphite powder magnetic impurity detection method comprises the following steps: placing a graphite powder sample in a non-magnetic container, then applying a vertical vibration force to the sample to make different substances in the sample move relative to each other; capturing the vibration response characteristics of different substances by a magnetic sensor, and then analyzing the impurity signal difference.

[0008] Further, a sound wave exciter is used to apply a vertical vibration force to the sample from directly below the non-magnetic container.

[0009] Further, the method further comprises a step of magnetizing the sample; the magnetizing direction of the sample and the detecting direction of the magnetic sensor are both vertical.

[0010] Further, the method for analyzing the impurity signal is: Time domain analysis: the signal collected by the magnetic sensor is a time domain signal; if the time domain signal only contains a pure graphite powder background noise signal, it indicates that the sample does not contain impurities; if it also contains other signals in addition to the pure graphite powder background noise signal, it indicates that the sample contains magnetic impurities; Frequency domain analysis: Fourier transform is performed on the time domain signal, the frequency spectrum peak value and harmonic characteristics are extracted, the pure graphite powder low-frequency response and the impurity high-frequency response are distinguished, transient signal analysis is performed by using short-time Fourier transform, the impurity short-time resonance event is captured, and the difference from the pure graphite powder vibration noise is distinguished based on the resonance event.

[0011] The application further provides a graphite powder body magnetic impurity detection device.

[0012] Further, the vibration unit is an acoustic wave exciter and is located directly below the non-magnetic container.

[0013] Further, the method further comprises a step of magnetizing the sample; the magnetizing direction of the sample and the detecting direction of the magnetic sensor are both vertical.

[0014] Further, the magnetizing unit is an adjustable electromagnet.

[0015] Further, the adjustable electromagnet is located below the non-magnetic container.

[0016] Further, the method further comprises a step of magnetizing the sample; the magnetizing direction of the sample and the detecting direction of the magnetic sensor are both vertical. Time domain analysis: the signal collected by the magnetic sensor is a time domain signal; if the time domain signal only contains a pure graphite powder background noise signal, it indicates that the sample does not contain impurities; if it also contains other signals in addition to the pure graphite powder background noise signal, it indicates that the sample contains magnetic impurities; Frequency domain analysis: Fourier transform is performed on the time domain signal, the frequency spectrum peak value and harmonic characteristics are extracted, the pure graphite powder low-frequency response and the impurity high-frequency response are distinguished, transient signal analysis is performed by using short-time Fourier transform, the impurity short-time resonance event is captured, and the difference from the pure graphite powder vibration noise is distinguished based on the resonance event.

[0017] The application has the following advantages: The application innovatively uses vertical direction vibration excitation, uses the superposition of gravity direction and vibration inertia to make graphite powder produce "looseness" and "relative motion"; magnetic impurities and graphite particles produce different vibration responses (phase difference, amplitude, harmonic component difference) under the action of inertia difference; this difference causes local magnetic field disturbance, becoming "distinguishable signal source" for subsequent magnetic detection; especially, vertical vibration force is applied to the sample from below, graphite powder particles are bounced up and loosened, graphite powder particles fall under the action of gravity, and then small range motion of graphite powder particles is completed, and disturbance to the magnetic field is generated.

[0018] The application saturates magnetic impurity particles by applying a strong magnetic field (>4000 Gs) in the vertical direction, and amplifies the difference between magnetization enhancement and magnetic response; magnetic impurities are subjected to magnetization torque in the external magnetic field, and small motion or spin occurs, thereby generating obvious magnetic field disturbance at the magnetic sensor; non-magnetic graphite has weak response, and the signal-to-noise ratio is significantly improved.

[0019] The application determines whether there is impurity by whether the magnetic field resonance signal of the magnetic impurity is generated in the time domain graph; the number and type of impurities are determined by the frequency of the position of different wave peaks (i.e. magnetic impurity signal peaks) in the frequency domain graph, different types of impurities have different resonance frequencies, which are used to distinguish the specific types of impurities. The relative content between the corresponding impurities is determined by the area of the wave peak in the frequency domain graph. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a time domain graph obtained by using the graphite body magnetic impurity detection method in embodiment 1 of the application; Figure 2 is a frequency domain graph obtained by using the graphite body magnetic impurity detection method in embodiment 1 of the application; Figure 3 is a structure schematic diagram of the graphite body magnetic impurity detection device in embodiment 2 of the application. DETAILED DESCRIPTION

[0021] To make the purpose, technical scheme and advantages of the embodiments of the application more clear, the technical scheme in the embodiments of the application will be described below in connection with the embodiments of the application. Obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0022] Embodiment 1 The graphite body magnetic impurity detection method provided in this embodiment includes the following steps: Step 1. Sample preparation Put the graphite powder in a non-magnetic container (plastic sample box), ensure that the powder is free or slightly compacted; The sample size can be adjusted according to the sensitivity of the sensor and the specifications of the vibration equipment, generally 10-50 g; The sample environment is to avoid external strong magnetic interference, and a shielding cylinder made of high magnetic permeability is used for magnetic field shielding.

[0023] Step 2. Excitation step Vibration excitation: The sample is subjected to vertical vibration by mechanical vibration (physical mechanical vibration and acoustic wave vibration, generally using an acoustic exciter, which can reduce the residual magnetism interference in mechanical parts). The vibration frequency range is set to any fixed frequency within the range of 10 Hz-200 Hz, (according to the bandwidth setting of the magnetic sensor, set to 37 Hz in the experiment), and attention should be paid to avoid the power frequency of 50 Hz, 100 Hz frequency point, to avoid the influence of power frequency on signal extraction. The vibration amplitude adjustment range is 100-500 microns, to avoid powder flying. In this embodiment, an acoustic exciter is used to emit sound waves upward below the non-magnetic container, and the sound waves make the graphite powder particles pop up and loosen from bottom to top, and then fall back under the action of gravity, so that the graphite powder particles produce small range movement, and then disturb the magnetic field.

[0024] Mechanical vibration can make the graphite powder particles produce small relative displacement and loosen, so that the magnetic impurity particles (such as iron filings, nickel filings, cobalt filings, etc.) mixed therein also produce relative motion under the action of inertia. Because the density and magnetism of these impurities are different from those of pure graphite powder, their vibration response characteristics (phase difference, amplitude, harmonic component) under the same frequency excitation are different, which leads to local magnetic field disturbance. This difference can be captured by a high-sensitivity magnetic sensor, so as to realize the differentiation of impurity signals. The vibration excitation of this embodiment must be in the vertical direction, otherwise it cannot achieve the effect. The sample has gravity, and the vertical vibration can make the compacted sample have a force in the vertical direction, so that the sample is loosened, and the graphite powder and impurities also produce relative motion under the action of inertia. If the vibration is in other directions, the effect is not good.

[0025] Magnetize the sample: Before applying vertical vibration force to the sample, the sample can also be magnetized, that is, a adjustable electromagnet magnetic field is placed in the vertical direction of the sample, and the magnetic field intensity is set to 4000 Gs or more, which can satisfy the saturation magnetization of 50g-100g sample.

[0026] The purpose of magnetization is to apply an external magnetization magnetic field to enhance the response difference between magnetic impurities and non-magnetic graphite. Magnetic particles will be affected by a magnetization torque in the magnetic field, causing spin or slight displacement, thereby generating a measurable magnetic field disturbance signal at the detection coil or magnetic sensor. Pure graphite powder has weak response due to its non-magnetic characteristics, so the signal contrast is significantly improved.

[0027] It should be noted that, in general, the content of magnetic impurities accounts for about ppb and ppm of the graphite powder particles, so the graphite powder containing magnetic impurities and the graphite powder not containing magnetic impurities have almost the same mass. Under mechanical vibration, the graphite powder containing magnetic impurities and the graphite powder not containing magnetic impurities will not be stratified, but the graphite powder can jump up and down slightly under the action of vertical vibration force, and this slight jump can form a local magnetic field disturbance, especially after magnetization, the local magnetic field disturbance caused by the magnetic impurities is more intense and easier to be captured by the magnetic sensor.

[0028] Step 3. Magnetic signal detection A high-sensitivity magnetic sensor such as an atomic magnetometer, a fluxgate or an AMR sensor is arranged above the sample at a distance of 5-20 mm from the sample, and the detection direction is vertical and the same as the magnetization direction of the sample. The acquisition frequency covers the vibration frequency and its high-order harmonics, and the sampling rate is 10-500 Hz; Step 4. Signal processing and differentiation Time domain analysis: compare the collected time domain signal with the pure graphite noise signal to determine whether there are magnetic impurities.

[0029] Frequency domain analysis: Fourier transform is performed on the collected magnetic signal to extract the frequency spectrum peak value and harmonic characteristics, and to distinguish the low-frequency response of pure graphite powder from the high-frequency response of impurities; short-time Fourier transform (STFT) is used for transient signal analysis to capture short-time resonance events of impurities; when the magnetic impurity particles are excited by vibration, their resonance response usually presents a short-time burst signal (transient peak), and different types of impurities have different time duration characteristics and frequency distribution. Through time-frequency analysis, these resonance events can be located on the time axis to distinguish them from the vibration noise of pure graphite powder and achieve accurate identification of impurities.

[0030] Specifically, as shown in Figure 1 , a time domain graph obtained after applying the method of the embodiment is shown in the figure. As can be seen from the figure, when the time domain signal is captured by the magnetic sensor 4, the three types of impurity signals have obvious differences in waveform and time axis due to different types of impurities. The vibration amplitude and frequency of the signal can be observed to change with time, as shown in Figure 1 The red, blue and green magnetic impurity signals in the middle have large differences in vibration amplitude and frequency. Then, frequency domain analysis is performed based on the time domain signal, as shown in Figure 2 .Figure 2 The spectrum peak and harmonic characteristics of each type of magnetic impurity are extracted Figure 2 The spectrum peak and harmonic characteristics of each type of magnetic impurity are extracted Figure 2 The spectrum peak and harmonic characteristics of each type of magnetic impurity are extracted

[0031] In summary, the key points of the graphite magnetic impurity detection method described in the embodiment are as follows: 1. Innovative application of vertical mechanical vibration excitation The vertical vibration excitation is innovatively adopted, and the gravitational direction and vibration inertia are superimposed to make the graphite powder body produce "looseness" and "relative motion"; the magnetic impurities and graphite particles produce different vibration responses (phase difference, amplitude, and harmonic component) under the action of inertia difference; this difference causes local magnetic field disturbance, which becomes a "distinguishable signal source" for subsequent magnetic detection; 2. Magnetic response difference amplification A strong magnetic field (>4000 Gs) is applied in the vertical direction to saturate the magnetic impurity particles; the magnetic impurity particles are subjected to a magnetic torque in the external magnetic field, and a small motion or spin occurs, thereby producing a significant magnetic field disturbance at the magnetic sensor; the non-magnetic graphite has a very weak response, and the signal-to-noise ratio is significantly improved.

[0032] 3. The embodiment determines whether there is an impurity by distinguishing the magnetic field resonance signal of the magnetic impurity from the noise signal in the time domain graph; the number and type of impurities are determined by the frequency of the position of the different wave peaks (i.e., magnetic impurity signal peaks) in the frequency domain graph; the resonance frequencies of different types of impurities are different, which are used to distinguish the specific types of impurities. The relative content between the corresponding impurities is determined by the area of the wave peak in the frequency domain graph.

[0033] Embodiment 2 The embodiment provides a graphite powder body magnetic impurity detection device applied to the graphite powder body magnetic impurity detection device of embodiment 1, as shown in Figure 3 The device includes a non-magnetic container 1 for containing a graphite powder sample 2, a vibration unit 3, a magnetic sensor 4, and a magnetization unit; the vibration unit 3 applies a vertical vibration force to the graphite powder sample 2, and the magnetic sensor 4 captures the vibration response characteristics of each different substance. The magnetization unit magnetizes the sample; the magnetization direction and the detection direction of the magnetic sensor are both vertical.

[0034] In the embodiment, the vibration unit 3 is a sound wave exciter. The magnetization unit is an adjustable electromagnet, which is located below the non-magnetic container 1. In the embodiment, the sound wave exciter is located directly below the non-magnetic container 1 and emits sound waves upward. Under the action of the sound waves from bottom to top, the graphite powder jumps and falls under the action of gravity, so that the graphite powder and the graphite powder containing impurities are displaced and the magnetic field is disturbed. The magnetic sensor 4 is located directly above the non-magnetic container 1.

[0035] For the convenience of testing, the non-magnetic container 1 is clamped by the mechanical arm 5 or the rotatable clamping piece in the embodiment, so that it is suspended. After the sample 2 is placed in the non-magnetic container 1, the mechanical arm 5 is rotated to rotate the non-magnetic container 1 to the magnetization unit for magnetization. After magnetization, the mechanical arm is rotated to rotate the non-magnetic container 1 between the vibration unit 3 and the magnetic sensor 4 (of course, the non-magnetic container can also be manually taken to the magnetization device, and after magnetization, it is taken to the vibration unit 3 and the sensor 4, and clamped by the clamping piece). The vibration unit is started to apply sound waves to the sample, and the magnetic sensor captures the vibration response characteristics of different substances, and then the analysis result is obtained. Of course, in order to avoid the influence of the magnetization device on the detection, the magnetization device and the sample detection position should be kept at a safe distance.

[0036] The above embodiments are only used to illustrate the technical solutions of the present application, but not limit it. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part of the technical features. The modification or replacement does not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for detecting magnetic impurities in graphite powder, characterized in that, The process includes the following steps: placing a graphite powder sample in a non-magnetic container, then applying a vertical vibration force to the sample to cause relative motion between different substances within the sample; capturing the vibration response characteristics of each different substance using a magnetic sensor, and then analyzing the differences in impurity signals.

2. The method for detecting magnetic impurities in graphite powder according to claim 1, characterized in that, A vertical vibration force is applied to the sample from directly below the non-magnetic container using an acoustic exciter.

3. The method for detecting magnetic impurities in graphite powder according to claim 1, characterized in that, Before applying the vertical vibration force, the sample is magnetized; the magnetization direction of the sample and the detection direction of the magnetic sensor are both vertical.

4. The method for detecting magnetic impurities in graphite powder according to any one of claims 1 to 3, characterized in that, The method for impurity signal analysis is as follows: Time-domain analysis: The signal collected by the magnetic sensor is a time-domain signal. If the time-domain signal only contains the noise signal of pure graphite powder, it means that the sample does not contain impurities. If it also contains other signals besides the noise signal of pure graphite powder, it means that the sample contains magnetic impurities. Frequency domain analysis: Fourier transform is performed on the time domain signal to extract the peak values ​​and harmonic features of the spectrum, distinguishing the low-frequency response of pure graphite powder from the high-frequency response of impurities. Transient signal analysis is performed using short-time Fourier transform to capture short-time resonance events of impurities. Based on the resonance events, the difference between the vibration noise of pure graphite powder and that of pure graphite powder is distinguished.

5. A device for detecting magnetic impurities in graphite powder applied to the detection method according to any one of claims 1 to 4, characterized in that, It includes a non-magnetic container for holding graphite powder samples, a vibration unit, and a magnetic sensor; the vibration unit applies a vertical vibration force to the graphite powder sample, and the magnetic sensor captures the vibration response characteristics of different substances.

6. The magnetic impurity detection device for graphite powder according to claim 5, characterized in that, The vibration unit is an acoustic exciter, located directly below the non-magnetic container.

7. The magnetic impurity detection device for graphite powder according to claim 5, characterized in that, It also includes a magnetization unit, which magnetizes the sample; the sample magnetization direction and the magnetic sensor detection direction are both vertical.

8. The magnetic impurity detection device for graphite powder according to claim 7, characterized in that, The magnetization unit is an adjustable electromagnet.

9. The magnetic impurity detection device for graphite powder according to claim 8, characterized in that, The adjustable electromagnet is located below the non-magnetic container.

10. The magnetic impurity detection device for graphite powder according to any one of claims 5 to 9, characterized in that, It also includes an impurity signal analysis unit; the impurity signal analysis unit acquires signals collected by a magnetic sensor, the signals including time-domain signals and magnetic signals; the method for impurity signal analysis is as follows: Time-domain analysis: The signal collected by the magnetic sensor is a time-domain signal. If the time-domain signal only contains the noise signal of pure graphite powder, it means that the sample does not contain impurities. If it also contains other signals besides the noise signal of pure graphite powder, it means that the sample contains magnetic impurities. Frequency domain analysis: Fourier transform is performed on the time domain signal to extract the peak values ​​and harmonic features of the spectrum, distinguishing the low-frequency response of pure graphite powder from the high-frequency response of impurities. Transient signal analysis is performed using short-time Fourier transform to capture short-time resonance events of impurities. Based on the resonance events, the difference between the vibration noise of pure graphite powder and that of pure graphite powder is distinguished.

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