Method for detecting ultra-trace ferromagnetic impurities in ternary cathode material

Abstract

The application relates to the technical field of battery material impurity detection, and particularly discloses a method for detecting ultra-trace ferromagnetic impurities in ternary positive electrode materials. The method comprises the following steps: placing a SERF atomic magnetometer in a magnetic shielding cylinder with a residual magnetic field noise peak-to-peak value lower than 50 nT; weighing a predetermined amount of the ternary positive electrode material to be detected, magnetizing the ternary positive electrode material, and then placing the ternary positive electrode material in the magnetic shielding cylinder; driving the ternary positive electrode material to be detected to move through the probe sensing area of the SERF atomic magnetometer, and collecting the magnetic field signal output by the SERF atomic magnetometer; and determining the content of ferromagnetic impurities in the ternary positive electrode material to be detected according to the magnetic field signal. The application can directly use the production line finished powder for room-temperature nondestructive rapid detection without sample pretreatment, realizes accurate detection of 1ppb ultra-low detection limit, and meets the industrialized batch strict quality control requirements of high-end high-nickel ternary positive electrode materials.

Description

Technical Field

[0001] This invention relates to the field of battery material impurity detection technology, and in particular to a method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials. Background Technology

[0002] Nickel-cobalt-manganese ternary cathode materials are the core materials of lithium-ion power batteries. Residual iron, iron oxides, stainless steel debris and other ferromagnetic metal impurities in the materials are the main sources of risk for lithium batteries to induce self-discharge, micro-short circuits and local overheating. In severe cases, they can cause battery thermal runaway and significantly reduce the safety and cycle life of the power battery.

[0003] As new energy vehicles develop towards high-end, high-safety, and long-life directions, high-nickel ternary cathode materials are gradually becoming the mainstream in the market, and the industry's control standards for ferromagnetic impurities are continuously tightening. Currently, the conventional internal control range for ferromagnetic impurities in mass-produced ternary materials is generally 10~50 ppb, while the control standards for ultra-high purity materials used in high-end export-grade and high-rate power batteries are as stringent as 1~10 ppb, with the highest impurity content in abnormally exceeding samples reaching the 300 ppb level. The peak value of the magnetic field signal generated by the magnetization of the above-mentioned ppb-level ultra-trace ferromagnetic impurities is generally within 200 pT, which belongs to an extremely weak low-frequency quasi-static magnetic field, placing extremely high demands on the sensitivity, accuracy, stability, and specificity of the detection methods.

[0004] Currently, mainstream testing technologies in the industry all have significant technical shortcomings and cannot simultaneously meet the testing requirements of ultra-high sensitivity, high specificity, non-destructive testing, speed, and mass production capability, as detailed below: 1. Vibrating Sample Magnetometer (VSM): Relies on mechanical vibration to collect magnetic signals, which has inherently high mechanical noise. The method detection limit is about 1 ppm, and it is only suitable for the detection of constant and trace ferromagnetic impurities. It cannot identify ultra-trace ferromagnetic impurities below 100 ppb, and cannot meet the ultra-high purity quality control requirements of high-end ternary materials.

[0005] 2. Magnetic Separation-ICP Method (GB / T 41704-2022 Arbitration Method): This is an industry-standard testing method that relies on strong magnetic rods to adsorb magnetic foreign matter, followed by acid elution, digestion, and quantitative ICP detection. This method has a testing process that takes 4-8 hours, and the pretreatment is extremely cumbersome. Furthermore, the magnetic separation structure has limited physical capture efficiency for ultrafine magnetic foreign matter smaller than 10μm, and some ultrafine impurities are easily missed. In addition, the magnetic separation process easily entrains a large number of harmless paramagnetic ternary matrix particles, magnetic inclusions, and powder agglomerates, ultimately leading to higher total magnetic foreign matter content results, poor consistency between results from different laboratories, and, being a destructive test, it cannot meet the rapid quality control needs of production lines.

[0006] 3. Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES): Although it can achieve ppb-level element detection capability, it requires strong acid digestion of samples, which is complicated in pretreatment, has a long detection cycle, and is destructive in detection process; moreover, it can only detect the total content of elements such as iron, chromium, nickel, and zinc, and cannot distinguish between the inherent paramagnetic metal components of the matrix and harmful ferromagnetic impurities that pose a risk to battery safety, resulting in poor detection specificity.

[0007] In summary, there are currently no certified reference materials for ferromagnetic impurities in ternary cathode materials that are compatible with the matrix, making it impossible to conduct accurate quantitative detection using conventional matrix-matched external standard calibration methods. Therefore, developing a method for detecting ultra-trace ferromagnetic impurities in ppb-level ternary cathode materials that is simple to prepare, highly sensitive, specific, stable, and compatible with batch quality control in production lines is a pressing technical challenge that needs to be addressed in this field. Summary of the Invention

[0008] Based on this, the purpose of this invention is to provide a method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials. This method requires no sample pretreatment and can directly use finished powder from the production line for rapid, non-destructive testing at room temperature. It achieves accurate detection at an ultra-low detection limit of 1 ppb, covering the entire detection range from 1 to 10 ppb for high-end ultra-high purity quality control, 10 to 50 ppb for conventional mass production internal control, and 50 to 300 ppb for exceeding the limit. This meets the stringent quality control requirements for industrial-scale production of high-end, high-nickel ternary cathode materials.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for detecting trace ferromagnetic impurities in ternary cathode materials, comprising the following steps: S1. Place the SERF atomic magnetometer (spin-free exchange relaxation atomic magnetometer) inside a magnetically shielded cylinder with residual magnetic field noise peak-to-peak value below 50 nT; S2. Prepare a predetermined amount of ternary cathode material to be tested, magnetize it, and then place it inside the magnetic shielding cylinder; S3. Drive the ternary cathode material to be tested to move through the probe sensing area of ​​the SERF atomic magnetometer and collect the magnetic field signal output by the SERF atomic magnetometer; S4. Determine the content of ferromagnetic impurities in the ternary cathode material to be tested based on the magnetic field signal.

[0010] This invention, through the timing control of pre-magnetization-field removal-residual magnetization acquisition, can specifically screen out harmful ferromagnetic impurities that can induce micro-short circuits, self-discharge, and thermal runaway risks in lithium batteries. In principle, it avoids the technical defects of magnetic separation-ICP method, which leads to high detection results and inability to distinguish between genuine and counterfeit harmful impurities due to paramagnetic matrix entrainment, magnetic inclusions, and powder mechanical agglomeration. The detection specificity and authenticity are greatly improved.

[0011] This invention achieves accurate detection based on the principle of remanence specificity of ferromagnetic materials: under the action of a preset DC magnetization field, all magnetic materials in the material can be magnetized; after the external magnetic field is removed, only ferromagnetic and ferrimagnetic impurities can retain a stable remanence signal within the detection period, while the induced magnetic field of paramagnetic materials such as ternary matrix and non-magnetic materials will rapidly decay to zero due to thermal motion.

[0012] The ultra-trace level of this invention specifically refers to the ultra-sensitive detection capability of this detection method with an ultra-low detection limit of 1 ppb, which can accurately identify ultra-high purity ultra-trace impurities in ternary materials ranging from 1 to 10 ppb. At the same time, it is compatible with the industry's conventional mass production internal control range of 10 to 50 ppb and the range of excessive impurities of 50 to 300 ppb, which is different from the limitation of traditional detection methods that can only detect impurities with medium to high content.

[0013] As a further improvement to the above-described solution of the present invention, after step S1 and before step S2, the following steps are also included: S20. Turn on the SERF atomic magnetometer. After stabilization, place the empty non-magnetic sample cell into the magnetic shielding cylinder and place it in the probe sensing area of ​​the SERF atomic magnetometer, and record the blank background signal B0.

[0014] As a further improvement to the above-mentioned solution of the present invention, in step S2, the preparation of a predetermined amount of ternary cathode material to be tested specifically involves: weighing a predetermined amount of ternary cathode material to be tested and filling it into a non-magnetic sample cell, holding the non-magnetic sample cell vertically to the table and tapping it several times, and then smoothing the surface.

[0015] In this invention, the non-magnetic sample cell is held perpendicular to the table and tapped several times (each tap is about 5cm high with uniform force) to allow the powder to settle naturally and evenly. After leveling, there are no obvious protrusions or depressions. Through the above standardized operating procedure, the relative deviation of the powder bulk density within the effective detection volume can be stably controlled within 3%, eliminating the need for additional density uniformity verification for each sample and meeting the repeatability requirements of industrial batch testing.

[0016] As a further improvement to the above-mentioned solution of the present invention, the non-magnetic sample cell is a disposable elliptical cylindrical PTFE non-magnetic sample cell with a major axis of 80 mm, a minor axis of 40 mm, a height of 20 mm, and an effective filling volume of 50 mL.

[0017] Because trace amounts of ferromagnetic impurities may remain on the inner wall of the non-magnetic sample cell from the previous test, subsequent test results may be falsely high. This invention preferably uses a disposable elliptical cylindrical PTFE non-magnetic sample cell, fundamentally eliminating the risk of cross-contamination, requiring no cleaning steps, further simplifying the testing process, and making it more suitable for industrial batch testing scenarios. If a reusable non-magnetic sample cell, such as one made of PCTFE trifluorochloroethylene, is used, it must be stipulated that after each test, the sample cell must be ultrasonically cleaned with anhydrous ethanol at least three times, dried at 105°C, and a blank test performed. Only after confirming that the residual signal is below the method detection limit (1 ppb) can the next test be performed.

[0018] As a further improvement of the above-mentioned scheme of the present invention, in step S2, the ternary cathode material to be tested is 20~50g, and the magnetization treatment is performed under a DC magnetization field with an intensity of 100~500mT for 30~120s, and the uniformity of the DC magnetization field is ≥99%.

[0019] As a further improvement to the above-mentioned scheme of the present invention, the ternary cathode material to be tested is 30g, the intensity of the DC magnetization field is 400mT, and the magnetization time is 60s. Gradient parameter optimization experiments verified that when the DC magnetization field intensity and magnetization time are lower than 400mT and 60s respectively, the ferromagnetic impurity domains in the material are not fully oriented, the magnetization has not reached a steady state, and the remanent magnetization signal increases significantly with increasing parameters. When the DC magnetization field intensity and magnetization time reach 400 mT and 60s respectively, most of the domains are oriented, the increase in the remanent magnetization signal is less than 6%, and the magnetization state approaches saturation. Further increasing the DC magnetization field intensity or extending the magnetization time contributes very little to the signal gain; instead, it increases the detection time, exacerbates equipment heating, and reduces system stability and device lifespan. Therefore, a DC magnetization field intensity of 400mT and a magnetization time of 60s are the optimal process parameters that balance magnetization sufficiency, detection repeatability, detection efficiency, and equipment reliability.

[0020] As a further improvement to the above-mentioned solution of the present invention, in step S2, within 10 seconds after the magnetization treatment is completed, the magnetized ternary cathode material to be tested is placed inside the magnetic shielding cylinder. The magnetization process is completed outside the magnetic shielding cylinder, and within 10 seconds after the magnetization is completed, the ternary cathode material to be tested is moved into the magnetic shielding cylinder for signal acquisition. This avoids residual magnetism attenuation and ensures that the ferromagnetic impurity domains inside the material are fully oriented along the direction of the external magnetic field, thereby obtaining a stable and reproducible residual magnetism state.

[0021] As a further improvement to the above-mentioned solution of the present invention, in step S1, the magnetic shielding cylinder is made of multi-layer permalloy composite, which can regulate the internal residual magnetic field noise to below 50 nT (peak-to-peak value), constructing a near-zero magnetic testing cavity, further reducing background noise, and improving the ability to identify ultra-trace signals and detection sensitivity. After subsequent digital filtering processing, the total environmental magnetic field noise of the system can be controlled below 1 pT.

[0022] As a further improvement to the above-described scheme of the present invention, in step S3, the ternary cathode material to be tested moves at a constant speed of 5~20 mm / s; preferably 10~15 mm / s, more preferably 12 mm / s. The sensitivity of the SERF atomic magnetometer is ≤15 fT / Hz. 1 / 2 It adopts a room temperature detection mode, has a low vibration structure, and does not require a low-temperature refrigeration system.

[0023] As a further improvement to the above-described solution of the present invention, step S4 specifically comprises: After performing a 0.1 Hz low-pass digital filter on the magnetic field signal B1, the ferromagnetic impurity content is calculated using the following formula:

[0024] In the formula, K is the response coefficient of the SERF atomic magnetometer.

[0025] The response coefficient of the SERF atomic magnetometer is determined by the equipment manufacturer before shipment using internal standard samples traceable to nationally certified pure iron standard materials. Under standardized process conditions defined in this invention—30g fixed sample, 400mT / 60s pre-magnetization, and a 12mm / s scan speed—the values ​​are uniformly calibrated and have legal metrological traceability. The typical average response coefficient of the SERF atomic magnetometer is 1.0~1.2pT / ppb, with coefficient deviations between different devices less than ±20%.

[0026] Ternary cathode materials themselves contain Ni 2+ Co 3+ Mn 4+ Plasma exhibits weak paramagnetism and generates a constant induced background signal under the influence of an external magnetic field. This invention employs a dual calibration system based on empty cell baseline calibration and enhanced by non-magnetic matrix blank subtraction: Empty cell baseline calibration: Before each test, under the same instrument parameters and scanning conditions as the test sample, the background signal of the empty non-magnetic sample cell is measured to subtract the inherent electronic noise background of the equipment, the residual magnetic field of the magnetic shielding cylinder, and the weak background interference of the sample cell itself.

[0027] Non-magnetic matrix blank subtraction: For special grades with significant differences in matrix paramagnetic background, a ternary cathode powder of the same grade as the test sample can be used as a non-magnetic matrix blank sample. This blank sample undergoes three-stage high-gradient deep magnetic separation purification (ferromagnetic impurity content <0.1 ppb), and its paramagnetic background signal is measured under identical detection conditions. Since the paramagnetism of ternary cathode materials of the same grade is mainly determined by their fixed stoichiometry and crystal structure, the difference in matrix paramagnetic background between different batches is extremely small, and its impact on the detection results is negligible. By subtracting the empty cell background signal from the original signal of the test sample, and, if necessary, additionally subtracting the blank matrix signal, the interference of the paramagnetic matrix background can be effectively eliminated, obtaining a pure magnetic response signal corresponding only to ferromagnetic impurities.

[0028] The specific operation method for blank subtraction of non-magnetic matrix is ​​as follows: (1) Blank sample preparation: Take ternary cathode powder of the same grade as the sample to be tested, and purify it by three-stage high gradient magnetic separation (magnetic field strength ≥1.2T per stage) to obtain a non-magnetic matrix blank sample with ferromagnetic impurity content <0.1ppb. The blank signal is confirmed to be stable by detection and verification using this method; (2) Blank signal measurement: Weigh 30g of non-magnetic matrix blank sample and detect it under the same detection conditions as the sample to be tested. The blank matrix signal B is measured. blank (3) Blank deduction calculation: Ferromagnetic impurity content = (B1 - B blank ) / K. It should be noted that only when the matrix paramagnetic background signal (B) is present... blank - When the increase of B0 relative to the empty cell background signal B0 exceeds 5%, non-magnetic matrix blank subtraction is required. This can be omitted for conventional grades.

[0029] The quantitative method employing instrument factory calibration plus empty cell baseline drift correction, as described in this invention, is a preferred solution for industrial-scale batch testing. The internal standard samples used by the equipment manufacturer are traceable to nationally certified Class I pure iron standard materials approved and issued by the State Administration for Market Regulation. Their values ​​possess legal metrological traceability, with a complete value transfer chain, complying with the "Metrology Law of the People's Republic of China" and related metrological technical specifications. This quantitative mode of instrument factory calibration plus empty cell baseline drift correction is a common metrological calibration practice in the analytical instrument industry for scenarios where commercially available certified standard materials are unavailable. Users can obtain accurate and stable quantitative results by strictly following the standardized process conditions specified in this invention. Comparative experiments with 18 batches of mass-produced samples verified the accuracy and reliability of this quantitative method. The correlation coefficient between the test results and the GB / T 41704-2022 magnetic separation-ICP method reached 0.94.

[0030] This invention employs a composite anti-interference system primarily based on multi-layer permalloy passive magnetic shielding and supplemented by a 0.1 Hz low-pass digital filter: the multi-layer permalloy composite forms a magnetic shielding cylinder, which can attenuate power frequency magnetic field interference within the test cavity by 10%. 4 The system achieves a noise reduction of more than 1 times, while simultaneously suppressing ultra-low frequency geomagnetic field fluctuations to below 50 nT (peak-to-peak), effectively isolating most environmental magnetic field interference at its source. Combined with a 0.1 Hz low-pass digital filter, residual stray interference signals above 0.1 Hz are further filtered out, reducing the system's equivalent magnetic field noise to below 1 pT. The selection of 0.1 Hz as the cutoff frequency is based on the fact that the sample scanning speed is 12 mm / s, and the characteristic frequency of the sample signal is approximately on the order of 0.1 Hz. This cutoff frequency can filter out irrelevant noise to the maximum extent while fully preserving the effective sample signal. Through the above-mentioned composite anti-interference measures, the total environmental magnetic field noise of the system can be controlled below 1 pT, meeting the detection requirements for ultra-trace ferromagnetic impurities at the 1 ppb level.

[0031] As a further improvement to the above-mentioned solution of the present invention, the ternary cathode material to be tested is one of NCM111, NCM523, NCM622, and NCM811; the ferromagnetic impurity is at least one of elemental iron, iron oxide, and stainless steel debris. And / or, the detection limit of the method for detecting ultra-trace ferromagnetic impurities in the ternary cathode material is ≤1 ppb (μg / kg, mass ratio), and the effective quantitative range is 1~300 ppb; within the effective quantitative range, the relative standard deviation (RSD) of the detection results is ≤5%, and the sample spike recovery rate is 95%~105%.

[0032] This invention, based on the principle of macroscopic magnetic response, is particularly suitable for the detection of trace ferromagnetic impurities in weakly paramagnetic matrix materials. For NCM series nickel-cobalt-manganese ternary cathode materials, the matrix paramagnetic signal is weak and stable, and will not significantly affect the detection results; therefore, this method has extremely high sensitivity and accuracy. For strongly paramagnetic matrix materials such as lithium iron phosphate, the paramagnetic background signal is much larger than the signal of ppb-level ferromagnetic impurities, and the paramagnetic properties of the matrix vary greatly between different batches. Background signal fluctuations cannot be eliminated by the fixed blank subtraction method, leading to a severe decrease in the signal-to-noise ratio and making accurate quantitative detection impossible. Therefore, this invention is currently not suitable for the detection of strongly paramagnetic matrix materials.

[0033] To ensure the long-term stability of the test results, the SERF atomic magnetometer needs to be calibrated monthly using standard samples. This calibration cycle is determined based on a long-term stability experiment conducted after six months of continuous operation of the equipment. Since the method of this invention exhibits good linear response characteristics within the 1–300 ppb quantitative range, single-point calibration is sufficient to meet the quantitative accuracy requirements.

[0034] In response to the lack of certified reference materials for ferromagnetic impurities in ternary matrices in the industry, this invention adopts a quantitative method of instrument factory coefficient calibration + empty cell baseline drift correction, which eliminates the dependence on commercially available matrix matching standard samples, establishes a universal quantitative mode applicable to scenarios without standard samples, and the measurement values ​​have legal metrological traceability, thus solving the long-standing problem of the lack of reference materials in the industry.

[0035] This invention enables rapid batch detection at room temperature without pretreatment, eliminating the need for any sample processing such as drying, grinding, or digestion. The detection time for a single sample is ≤10 min, perfectly meeting the high-volume, rapid quality control requirements of lithium battery production lines. At the same time, the equipment does not require cryogenic refrigeration, reducing the overall cost by more than 70% compared to the SQUID superconducting magnetometer, thus promoting the transition of ppb-level ferromagnetic impurity detection technology from laboratory research to industrial mass production applications.

[0036] This invention detects a relative standard deviation (RSD) ≤ 5% and a spiked recovery rate that is stable at 95%~105%, meeting the requirements for internal control and third-party testing of high-end materials. Attached Figure Description

[0037] Figure 1 A flowchart of a method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials provided by the present invention; Figure 2 This is a schematic diagram showing the signal distribution and 3σ detection limit of the blank sample in 11 repeated tests according to the present invention; Figure 3 A bar chart comparing the detection limits of ferromagnetic impurities using different detection methods; Figure 4 This is a schematic diagram showing the correlation between the detection results of the method of this invention and the magnetic separation-ICP method in GB / T 41704-2022; Figure 5 The time response curves of remanence signal of ternary matrix samples under different magnetization parameters are shown. Figure 6 The graph shows the response curves of the remanence signal of the standard impurity sample as a function of time under different magnetization parameters. Figure 7 This is a linear calibration curve of the magnetic response signal versus the content of ferromagnetic impurities. Detailed Implementation

[0038] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0040] Reference Figure 1 This embodiment provides a method for detecting trace ferromagnetic impurities in ternary cathode materials, which includes the following steps: S1. Low magnetic environment construction: A commercially available SERF atomic magnetometer (sensitivity ≤15 fT / Hz) will be used for magnetic measurement. 1 / 2 The sample cell is placed inside a magnetic shielding cylinder (formed by four layers of permalloy with built-in demagnetizing coils) to magnetically shield the external magnetic field of the SERF atomic magnetometer's working environment. After turning on the SERF atomic magnetometer and the device stabilizes after 30 minutes, an empty disposable elliptical cylindrical PTFE non-magnetic sample cell is placed inside the magnetic shielding cylinder and placed within the probe sensing area of ​​the SERF atomic magnetometer. The background signal is continuously collected for 5 seconds, and the average value is taken as the recorded blank background signal B0.

[0041] Before leaving the factory, the SERF atomic magnetometer has been calibrated using internal standard samples traceable to nationally certified Class I pure iron standard materials, and its response coefficient is fixed in the equipment control system. In this embodiment, the SERF atomic magnetometer has a response coefficient K = 1.1 pT / ppb under a 30g sample weight condition.

[0042] In this embodiment, the major axis of the disposable elliptical cylindrical PTFE non-magnetic sample cell is 80mm, the minor axis is 40mm, the height is 20mm, and the effective filling volume is 50mL.

[0043] S2. Sample standardization packaging: Accurately weigh 30g of the ternary cathode material to be tested, and evenly fill it into a disposable elliptical cylindrical PTFE non-magnetic sample cell. Hold the non-magnetic sample cell vertically to the table and tap it 3 times (each tap is about 5cm high and the force is even). Then, scrape the surface smooth. There should be no obvious bulges or depressions after smoothing.

[0044] S3. Pre-magnetization: Place the non-magnetic sample cell containing the ternary cathode material to be tested in the magnetization device, apply a uniform DC magnetization field of 400mT, and magnetize for 60s; within 10s after magnetization, move the non-magnetic sample cell into the magnetic shielding cylinder. Before placing the non-magnetic sample cell in, turn on the SERF atomic magnetometer 30min in advance to stabilize the equipment.

[0045] The magnetization process is completed outside the magnetic shielding cylinder. The magnetization device is a C-type DC electromagnet, which is driven by a constant current power supply to generate a uniform DC magnetization field. The magnetic pole spacing is adjustable. The non-magnetic sample cell is placed in the uniform field area between the two poles of the C-type DC electromagnet. Preferably, the magnetic field uniformity in the effective magnetization area is ≥99.2%.

[0046] S4. Uniform speed scanning signal acquisition: The non-magnetic sample cell is driven by the uniform speed transmission mechanism of the magnetic measuring device to pass through the probe sensing area of ​​the SERF atomic magnetometer at a constant speed of 12 mm / s, and the magnetic field signal B1 is continuously acquired.

[0047] S5. Perform a 0.1Hz low-pass digital filter on the magnetic field signal to remove the blank background signal and obtain the pure magnetic response signal. Calculate the ferromagnetic impurity content quantitatively using the following formula:

[0048] In this embodiment, the 0.1 Hz low-pass digital filtering process is specifically as follows: a fourth-order Butterworth IIR low-pass digital filter is used with a cutoff frequency of 0.1 Hz (-3dB attenuation point) and an effective data sampling rate of 10 Hz. The filtering is performed in real time in the device's built-in DSP, digitally low-pass filtering the original magnetic field signal output by the SERF atomic magnetometer to filter out high-frequency noise interference above 0.1 Hz and retain the effective magnetic response signal generated by the sample scanning. The filtered signal is then zero-phase corrected to ensure the accurate peak position of the ferromagnetic impurity signal. Finally, the filtered magnetic field signal B1 is output for quantitative calculation.

[0049] Detection limit verification: In step S1, 11 independent parallel tests were performed on an empty, disposable elliptical cylindrical PTFE non-magnetic sample cell. Figure 2 As shown, the measured blank background signals are as follows: 0.15 pT, 0.62 pT, -0.20 pT, 0.38 pT, 0.55 pT, -0.12 pT, 0.42 pT, 0.08 pT, 0.29 pT, 0.51 pT, -0.05 pT (Note: The blank background signal is the differential signal relative to the instrument baseline zero point, and positive and negative fluctuations are allowed). The calculated average blank background signal is ≈0.23 pT, and the standard deviation σ is ≈0.29 pT. Using 3σ as the signal corresponding to the detection limit, substituting it into the response coefficient K, the detection limit is calculated as: Detection limit = (3 × 0.29) ÷ 1.1 ≈ 0.79 ppb. After rounding, the method detection limit is ≤1 ppb.

[0050] The comparison chart of the detection limits of ferromagnetic impurities between the method of this invention and existing detection methods is shown in the figure below. Figure 3 As shown, from Figure 3 It can be seen that the detection limit of the method of the present invention is improved by three orders of magnitude.

[0051] It should be noted that, to ensure the long-term stability of the test results, the SERF atomic magnetometer needs to be calibrated monthly using a standard sample. The standard sample can be a ternary cathode powder with a known ferromagnetic impurity content, or a stainless steel particle standard sample of known mass (simulating typical ferromagnetic impurities in lithium batteries). Details are as follows: Under standardized testing conditions, at least three parallel tests are performed on the standard sample, and the average magnetic field signal value Bstd is taken. The new response coefficient K' is calculated according to the formula: K'=(Bstd-B0) / Cstd, where Cstd is the nominal ferromagnetic impurity content of the standard sample and B0 is the empty cell background signal. Compare the measured value K' with the nominal value K. If the deviation is ≤ ±5%, no adjustment is needed. If the deviation is > ±5%, write the measured value K' into the parameter storage unit of the equipment control system, replace the original response coefficient, and complete the calibration update.

[0052] The method of the present invention will be further described below with reference to specific embodiments.

[0053] Example 1: Sample testing of NCM523 from a conventional ternary production line This embodiment uses mass-produced NCM523 ternary cathode powder as the sample to be tested to illustrate the method of the present invention.

[0054] Precision Verification: In this embodiment, six independent parallel tests were performed on the same batch of NCM523 samples. The pure magnetic response signals were 17.71 pT, 18.04 pT, 18.26 pT, 18.48 pT, 17.93 pT, and 18.26 pT, respectively; the ferromagnetic impurity contents were measured to be 16.1 ppb, 16.4 ppb, 16.6 ppb, 16.8 ppb, 16.3 ppb, and 16.6 ppb, respectively. The relative standard deviation (RSD) of the six parallel tests was calculated to be 1.5%.

[0055] Spike recovery experiment: (1) Preparation of standard suspension: Take 100 mg of high-purity iron powder (purity 99.99%, particle size 100~200 nm), add anhydrous ethanol and 0.1% polyethylene glycol dispersant, sonicate for 30 min, and make up to 100 mL to prepare a 1 mg / mL stock solution; take 1 mL of the stock solution, dilute and make up to 100 mL to obtain a 10 μg / mL standard working suspension.

[0056] (2) Grouping and spiking: Take 3 30g samples of the original NCM523 to be tested and divide them into group A (blank group), group B (low spiking group), and group C (high spiking group).

[0057] (3) Spiking operation: Group B was added with 18 μL of standard working suspension, with a theoretical spiking amount of 6 ppb; Group C was added with 36 μL of standard working suspension, with a theoretical spiking amount of 12 ppb; Group A was not treated.

[0058] (4) Sample post-treatment: After the three groups of samples were stirred evenly, they were placed in a vacuum oven at 60℃ and dried for 30 min to remove ethanol. Note: This drying is only to remove the spiked solvent. No pretreatment is required for the mass-produced finished powder.

[0059] (5) Detection and calculation: Group A: Pure magnetic response signal 18.2 pT, total impurity content 16.5 ppb; Group B: Pure magnetic response signal 24.5 pT, total impurity content 22.3 ppb, recovery rate 96.7%; Group C: Pure magnetic response signal 31.0 pT, total impurity content 28.2 ppb, recovery rate 97.5%.

[0060] Example 2: Sample testing of NCM811 high-nickel ternary lithium-ion ... This embodiment uses mass-produced NCM811 high-nickel ternary cathode powder (median particle size D50≈12μm, specific surface area≈0.8m² / g) as the sample to be tested to illustrate the method of the present invention.

[0061] In this embodiment, six independent parallel tests were performed on the same batch of NCM811 samples. The pure magnetic response signals were 4.1 pT, 3.9 pT, 4.2 pT, 4.0 pT, 3.8 pT, and 4.1 pT, respectively. The measured ferromagnetic impurity contents were 3.7 ppb, 3.5 ppb, 3.8 ppb, 3.6 ppb, 3.5 ppb, and 3.7 ppb, respectively, meeting the internal control standard of <10 ppb for high-end NCM811 materials. The relative standard deviation (RSD) of the six parallel tests was calculated to be 3.8%.

[0062] Spike recovery experiment: (1) Preparation of standard suspension: Take 100 mg of high-purity iron powder (purity 99.99%, particle size 100~200 nm), add anhydrous ethanol and 0.1% polyethylene glycol dispersant, sonicate for 30 min, and make up to 100 mL to prepare a 1 mg / mL stock solution; take 1 mL of the stock solution, dilute and make up to 100 mL to obtain a 10 μg / mL standard working suspension.

[0063] (2) Grouping and spiking: Take 3 30g samples of the original NCM811 to be tested and divide them into group A (blank group), group B (low spiking group), and group C (high spiking group).

[0064] (3) Spiking operation: Group B was added with 18 μL of standard working suspension, with a theoretical spiking amount of 6 ppb; Group C was added with 36 μL of standard working suspension, with a theoretical spiking amount of 12 ppb; Group A was not treated.

[0065] (4) Sample post-treatment: After the three groups of samples were stirred evenly, they were placed in a vacuum oven at 60℃ and dried for 30 min to remove ethanol. Note: This drying is only to remove the spiked solvent. No pretreatment is required for the mass-produced finished powder.

[0066] (5) Detection calculation: The calculated recoveries of 6ppb and 12ppb spikes were 96.8% and 97.6%, respectively.

[0067] For NCM811 high-nickel ternary cathode material, the method of this invention can effectively detect 3.7 ppb of impurities; VSM vibrating sample magnetometer and fluxgate magnetometer cannot effectively detect this ultra-low content impurity.

[0068] Example 3: Linear calibration of magnetic response signal and ferromagnetic impurity content This embodiment verifies the linear response characteristics of the method of the present invention within the quantitative range of 1~300 ppb.

[0069] Experimental methods: (1) Preparation of blank matrix: NCM523 ternary cathode powder was taken and purified by three-stage high gradient magnetic separation to obtain blank matrix powder with ferromagnetic impurity background <0.1ppb; (2) Preparation of standard suspension: Accurately weigh 30μm stainless steel particles (simulating typical ferromagnetic impurities in lithium batteries), prepare a standard suspension containing 0.1% polyethylene glycol dispersant with anhydrous ethanol, and dilute stepwise to obtain a standard working suspension with a concentration of 0.1μg / mL; (3) Preparation of calibration series: Weigh 6 portions of 30g matrix blank powder and add different volumes of standard working suspension to prepare a series of calibration samples with ferromagnetic impurity contents of 0 ppb, 10 ppb, 50 ppb, 100 ppb, 200 ppb and 300 ppb respectively. Prepare 3 portions of each concentration in parallel. After adding the spiking, stir evenly and dry under vacuum at 60℃ to remove ethanol. (4) Under standardized testing conditions, samples of each concentration were tested sequentially, and the peak value of the pure magnetic response signal was recorded.

[0070] Experimental results: such as Figure 4 As shown, a linear regression was performed with the ferromagnetic impurity content as the abscissa and the pure magnetic response signal as the ordinate, yielding the calibration curve equation y = 1.10x, and the linear correlation coefficient R. 2A slope ≥0.999 indicates that this method exhibits good linear response characteristics in the range of 1~300 ppb. This slope of 1.10 pT / ppb is the response coefficient K of the SERF atomic magnetometer.

[0071] Example 4: Comparative Validation of Detection Methods for 18 Batches of NCM523 Mass-Produced Samples This embodiment uses 18 batches of mass-produced NCM523 samples as the test samples to illustrate the method of the present invention.

[0072] Eighteen batches of mass-produced NCM523 samples were selected as test samples and simultaneously detected using the method of this invention and the GB / T41704-2022 magnetic separation-ICP method. The instrument response coefficient K=1.1 pT / ppb. The test results are shown in Table 1.

[0073] Table 1 Detection results of different methods

[0074] Table 1 Results Analysis: like Figure 5 As shown, the Pearson correlation coefficient R between the test results of the method of this invention and the test results of the magnetic separation-ICP method in GB / T 41704-2022 is 0.94, indicating that the two methods have a good correlation. The correlation is particularly significant in the high-concentration range (≥50 ppb), with R... 2 ≥0.92.

[0075] For samples with significant deviations, such as NCM523-1 and NCM523-4, a second test was conducted using a 1.2T high-gradient magnetic separation method with an extended separation time. The results from the magnetic separation-ICP method were found to be significantly closer to those from the method described in this invention, confirming that the deviation was mainly due to the insufficient capture efficiency of the magnetic separation-ICP method for ultrafine ferromagnetic impurities. The discrepancy between the two numerical values ​​is primarily due to the following reasons: 1. Different detection targets: GB / T 41704-2022 Magnetic separation-ICP method is a common arbitration method in the industry. Its detection result is the total content of four elements, iron, chromium, nickel and zinc, in the magnetically separated concentrate, which is converted into the total content of magnetic foreign matter as defined by the standard. This includes some paramagnetic ternary matrix particles that are entrained. However, the method of this invention is based on the principle of remanent magnetism specificity and only quantifies ferromagnetic impurities that can induce the risk of micro short circuits in batteries. The detection targets of the two are fundamentally different.

[0076] 2. Differences in ultrafine impurity capture capability: As shown in the above retest results, the method of this invention, based on the principle of overall magnetic response, is not limited by the particle size of impurities and can comprehensively capture all ferromagnetic impurity signals. In contrast, conventional magnetic separation methods have limited capture efficiency for ultrafine foreign objects smaller than 10 μm, and the capture rate decreases significantly with decreasing particle size. This is the main reason why the detection results of this invention are higher for some samples in the low-to-medium concentration range. Therefore, the results of the method of this invention can more comprehensively reflect the true safety status of the sample.

[0077] 3. Matrix Entrainment Effect in High-Concentration Areas: For severely excessive samples such as NCM523-8, the magnetic separation-ICP method measured 300.94 ppb, while the method of this invention measured 174.00 ppb, a relatively low value. This is because the magnetic impurity content is high at high concentrations, and the magnetic agglomerates formed during magnetic separation easily entrain fine particles of the ternary matrix, which are also collected. The ternary matrix itself contains a large amount of nickel, which is also measured by ICP after acid hydrolysis, leading to a systematic overestimation of the total amount of magnetic foreign matter. In contrast, the method of this invention is based on the specific response of remanent magnetization, generating a signal only for ferromagnetic materials with remanent magnetization, unaffected by interference from paramagnetic matrix entrainment, resulting in more specific quantification.

[0078] The above results show that the method of the present invention has a good correlation with the industry-standard arbitration method, and has significant advantages in detection specificity, ultrafine impurity capture capability and industrial detection efficiency. Moreover, the method of the present invention can complete single-sample detection within 10 minutes, which is much faster than the 4-8 hour detection cycle of the magnetic separation-ICP method, fully meeting the requirements for rapid early warning of production line anomalies and meeting the stringent requirements for quality control of ferromagnetic impurities in ternary cathode materials.

[0079] Example 5: Experiment on the Influence of Different Magnetization Parameters on Remanence Signal This embodiment aims to investigate the influence of different magnetization field strengths and magnetization times on the background signal of the ternary matrix and the remanent magnetization signal of harmful ferromagnetic impurities, in order to determine the optimal process parameters that balance detection sensitivity, signal-to-noise ratio, equipment stability and industrialization efficiency.

[0080] Experimental samples and conditions: 1. Ternary matrix samples: Three different batches of mass-produced NCM523 ternary cathode powder were used for field strength testing at 200mT, 400mT, and 600mT, respectively. 2. Standard impurity samples: 30μm stainless steel particles (typical harmful ferromagnetic impurities in lithium batteries) were used for the above field strength tests. 3. Uniform conditions: All tests used the same SERF atomic magnetometer, with a sample weight of 30g, a disposable elliptical cylindrical PTFE non-magnetic sample cell, and the same signal acquisition parameters.

[0081] The results are shown in Tables 2 and 3. Figure 6-7As shown.

[0082] Table 2. Effects of different magnetization parameters on the remanent magnetization signal of NCM523 ternary matrix (unit: pT)

[0083] Table 3. Effects of different magnetization parameters on the remanent magnetization signal of 30μm stainless steel particles (unit: pT)

[0084] It should be noted that in Tables 2 and 3, the magnetization 0s signal is the inherent background signal of the sample measured before the pre-magnetization field is applied, which includes the equipment background and environmental noise.

[0085] Analysis of results in Tables 2 and 3: 1. Rapid saturation characteristics of ferromagnetic impurities: For 30μm stainless steel particles, when the magnetization reaches 400 mT, the signal reaches its maximum value (13.42 pT) at 60 s. Extending the magnetization time to 120 s, the signal only fluctuates to 13.51 pT and remains relatively stable. When the magnetization increases to 600 mT, the maximum signal value is only 13.37 pT, essentially the same as at 400 mT. Furthermore, the signal slightly decreases when the magnetization time is extended to 120 s, presumably due to system drift caused by the heating of the electromagnet under high field conditions. This indicates that 400 mT is sufficient to bring most ferromagnetic impurities to magnetic saturation, resulting in the maximum impurity signal response.

[0086] 2. Slow polarization characteristics of the paramagnetic matrix: For the NCM523 ternary matrix, the signal increased slowly and continuously with magnetization time under all test field strengths, and had not yet reached a plateau at 120s, which is a typical physical characteristic of paramagnetic materials. When the magnetization intensity increased from 400mT to 600 mT, the signal at 60s magnetization increased by 28% (15.03pT → 19.26pT), and the signal continued to increase from 60 to 120s (19.26 pT → 19.51pT), indicating that this increase came entirely from the polarization of the paramagnetic matrix, rather than from the signal of harmful ferromagnetic impurities.

[0087] 3. Signal-to-noise ratio (SNR) optimization analysis: At 600 mT, the increase in total signal is entirely due to the harmless matrix background, which leads to a significant decrease in SNR and is detrimental to the detection of ppb-level ultra-trace ferromagnetic impurities. At 400 mT, however, it ensures that harmful ferromagnetic impurities are fully magnetized to obtain the maximum signal while suppressing matrix paramagnetic background interference to the maximum extent, thus achieving the highest detection SNR.

[0088] 4. Equipment reliability and efficiency analysis: Increasing the magnetization intensity to 600mT will more than double the power consumption of the equipment and significantly increase the heat generation, which will reduce the stability of the system in long-term operation; extending the magnetization time to 120s will increase the signal of harmful impurities by less than 1%, but the detection cycle will be doubled, which is not conducive to industrial batch detection.

[0089] The three different batches of NCM523 samples all exhibited completely consistent patterns, proving that the above conclusions are universal and independent of the sample batch. Therefore, a magnetization intensity of 400 mT and a pre-magnetization time of 60 s are the optimal process parameters that balance the sufficiency of impurity magnetization, the signal-to-noise ratio of detection, equipment reliability, and detection efficiency.

[0090] Example 6: Validation of the suitability of strongly paramagnetic matrix materials for testing This embodiment aims to verify the applicability of the method of the present invention under different matrix backgrounds. Lithium iron phosphate (LFP) strongly paramagnetic matrix material was selected for testing. All process conditions (400mT / 60s pre-magnetization, 12mm / s scan speed, instrument response coefficient K=1.1 pT / ppb) were completely consistent with the testing conditions for NCM ternary materials. Simultaneously, the total content of magnetic impurities was tested using the GB / T 41704-2022 magnetic separation-ICP method, and six representative sets of data are shown in Table 4.

[0091] Table 4 Detection results of magnetic impurities in lithium iron phosphate

[0092] It should be noted that the formula for calculating the relative deviation in Table 4 is as follows:

[0093] The high sensitivity and quantitative accuracy of the method in this invention rely on the low and stable background signal of the weakly paramagnetic matrix to ensure the effective identification of ultra-trace ferromagnetic impurity signals. The detection results for lithium iron phosphate matrix materials are analyzed as follows: 1. Significant fluctuations in matrix background signal, interference cannot be eliminated: Lithium iron phosphate itself is a strongly paramagnetic material, and the background magnetic signal generated by its matrix ranges from 3.25 to 80.67 pT / g, which is 1 to 3 orders of magnitude higher than that of NCM ternary materials (approximately 0.1 to 0.3 pT / g); moreover, the matrix paramagnetism of different batches of lithium iron phosphate varies significantly, and this fluctuation is not related to the content of magnetic foreign matter, so the interference cannot be eliminated by the fixed baseline subtraction system of the method of this invention.

[0094] 2. Samples with conventional content cannot meet quantitative accuracy requirements: For conventional lithium iron phosphate samples (LFP-1, LFP-2, LFP-3) with magnetic foreign matter content below 100 ppb, the calculation results of the method of this invention deviate significantly from the results of the magnetic separation-ICP method in GB / T 41704-2022, exceeding the acceptable range for industrial quality control, thus failing to achieve accurate quantification. Taking the LFP-1 sample as an example, its magnetic foreign matter content is only 33.24 ppb, but the matrix background signal is as high as 80.39 pT / g, resulting in a deviation of more than 65 times in the calculation results of the method of this invention, which lacks industrial quantitative value.

[0095] 3. Low-bias results lack repeatability: For some samples with abnormally low matrix paramagnetism (such as LFP-4 and LFP-5), although the calculated bias is relatively small, this bias is an accidental result of the matrix background in a specific batch. The fluctuation of matrix paramagnetism background between different batches can be several to tens of times, lacking repeatability and unable to be stably applied to industrial quality control scenarios. In this experiment, only 2 out of 20 batches of samples had a bias of less than 50%, while the biases of the remaining 18 batches all exceeded 100%, fully verifying that this method has no stable quantitative capability for strongly paramagnetic matrix materials.

[0096] 4. Severely excessive samples can only achieve rough qualitative identification: For severely excessive samples with magnetic foreign matter content exceeding 2000 ppb (such as LFP-6), although this method can detect obvious magnetic signals, the calculation results still deviate from the results of the magnetic separation-ICP method by as much as 205%. It can only achieve rough qualitative identification of whether it is severely excessive and cannot meet the quantitative accuracy requirements of industrial quality control.

[0097] In summary, the method of this invention is based on the principle of macroscopic magnetic response, and its ability to detect ultra-trace impurities relies on the low and stable background interference of a weakly paramagnetic matrix. For strongly paramagnetic matrix materials such as lithium iron phosphate, the intensity and fluctuation characteristics of the matrix background signal do not meet the quantitative prerequisites of this invention; therefore, this invention is not suitable for the detection of such materials. The core application scenario of this invention is weakly paramagnetic matrices such as NCM series nickel-cobalt-manganese ternary cathode materials, enabling stable and accurate quantitative detection in the range of 1–300 ppb.

[0098] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0099] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials, characterized in that, It includes the following steps: S1. Place the SERF atomic magnetometer inside a magnetically shielded cylinder with residual magnetic field noise peak-to-peak value below 50 nT; S2. Prepare a predetermined amount of ternary cathode material to be tested, magnetize it, and then place it inside the magnetic shielding cylinder; S3. Drive the ternary cathode material to be tested to move through the probe sensing area of ​​the SERF atomic magnetometer and collect the magnetic field signal output by the SERF atomic magnetometer; S4. Determine the content of ferromagnetic impurities in the ternary cathode material to be tested based on the magnetic field signal.

2. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 1, characterized in that, In step S1, the magnetic shielding cylinder is made of multilayer permalloy composite. And / or, after step S1 and before step S2, the following steps are also included: S20. Turn on the SERF atomic magnetometer. After stabilization, place the empty non-magnetic sample cell into the magnetic shielding cylinder and place it in the probe sensing area of ​​the SERF atomic magnetometer, and record the blank background signal B0.

3. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 1, characterized in that, In step S2, preparing a predetermined amount of ternary cathode material to be tested specifically involves: Weigh out a predetermined amount of the ternary cathode material to be tested and fill it into a non-magnetic sample cell. Hold the non-magnetic sample cell vertically to the table and tap it several times, then scrape the surface smooth.

4. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 2 or 3, characterized in that, The non-magnetic sample cell is a disposable elliptical cylindrical PTFE non-magnetic sample cell with a major axis of 80 mm, a minor axis of 40 mm, a height of 20 mm, and an effective filling volume of 50 mL.

5. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 1, characterized in that, In step S2, the ternary cathode material to be tested is 20~50g, and the magnetization treatment is performed under a DC magnetization field with an intensity of 100~500mT for 30~120s, and the uniformity of the DC magnetization field is ≥99%.

6. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 5, characterized in that, The ternary cathode material to be tested is 30g, the intensity of the DC magnetization field is 400mT, and the magnetization treatment time is 60s.

7. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 1, characterized in that, In step S2, within 10 seconds after the magnetization process is completed, the magnetized ternary cathode material to be tested is placed inside the magnetic shielding cylinder.

8. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 1, characterized in that, In step S3, the ternary cathode material to be tested moves at a constant speed of 5~20 mm / s; the sensitivity of the SERF atomic magnetometer is ≤15 fT / Hz. 1 / 2 It adopts a room temperature detection mode.

9. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 2, characterized in that, Step S4 is as follows: After performing a 0.1 Hz low-pass digital filter on the magnetic field signal B1, the ferromagnetic impurity content is calculated using the following formula: In the formula, K is the response coefficient of the SERF atomic magnetometer.

10. The method for detecting ultra-trace ferromagnetic impurities in ternary cathode materials according to claim 1, characterized in that, The ternary cathode material to be tested is one of NCM111, NCM523, NCM622, and NCM811; the ferromagnetic impurity is at least one of elemental iron, iron oxide, and stainless steel debris. And / or, the detection limit of the ultra-trace ferromagnetic impurity detection method for the ternary cathode material is ≤1 ppb, and the effective quantitative range is 1~300 ppb; within the effective quantitative range, the relative standard deviation (RSD) of the detection result is ≤5%, and the sample spike recovery rate is 95%~105%.

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