Lithium ion battery lithium precipitation determination method based on characteristic frequency disturbance and impedance response

Abstract

The invention relates to the technical field of safety detection and state monitoring of lithium ion batteries, in particular to a lithium ion battery lithium precipitation judgment method based on characteristic frequency disturbance and impedance response. The method comprises the following steps: placing a lithium ion battery to be detected in an incubator for standing, and then starting a conventional charging and discharging process; synchronously applying a characteristic single-frequency sine alternating current signal in a conventional charging and discharging process, and carrying out a single-frequency impedance test; impedance real part parameters in the single-frequency impedance test are collected according to set intervals; drawing an SOC relation graph of the impedance real part value and the charging and discharging state of charge according to the impedance real part parameters; and a sudden change inflection point of an impedance real part in the SOC relation graph is identified, and battery lithium precipitation determination is completed. Therefore, the lithium separation boundary of the battery cell under different working conditions is drawn in a mode that equipment is simple, the battery does not need to be disassembled and the lithium separation starting point is accurately recognized through dynamic response of the characteristic frequency, and the method can be suitable for the lithium ion battery cell under different working conditions.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery safety testing and condition monitoring technology, and in particular to a method for determining lithium plating in lithium-ion batteries based on characteristic frequency perturbation and impedance response. Background Technology

[0002] Lithium plating during the charging and discharging process of lithium-ion batteries can seriously affect the battery's safety, cycle life, and electrochemical performance. Therefore, accurately identifying the starting point of lithium plating and drawing lithium plating boundary maps under different operating conditions is a key requirement in the field of lithium-ion battery safety testing and condition monitoring.

[0003] In existing technologies, CN120522584A proposes a dynamic frequency sweep impedance in-situ detection method, which obtains lithium plating signals by applying a sinusoidal AC signal within a certain frequency range to the cell during charging and discharging. However, this method uses a frequency sweep mode, which requires a long time to complete the full-band scan in dynamic charging and discharging scenarios. This not only makes it difficult to adapt to the rapid detection requirements in actual working conditions, but also makes it impossible to capture the starting point of the lithium plating reaction in time due to the long test cycle, resulting in a lag in lithium plating identification. Another existing technology, CN114578243A, measures the limiting current of the battery map using a pulse method. Although it can achieve performance boundary assessment under some working conditions, this method does not focus on the accurate identification of the lithium plating starting point and cannot systematically cover the entire working range consisting of temperature, rate, and state of charge (SOC), making it difficult to form a complete lithium plating boundary map.

[0004] In summary, existing technologies generally suffer from technical defects such as insufficient accuracy in identifying the starting point of lithium plating under dynamic operating conditions, low testing efficiency, and inability to comprehensively cover multiple operating conditions to draw a complete lithium plating boundary map. They fail to address the core need for rapid, accurate, and comprehensive monitoring of the lithium plating status of lithium-ion batteries in engineering applications. Summary of the Invention

[0005] Therefore, it is necessary to provide a lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response to solve at least one of the above-mentioned technical problems.

[0006] To achieve the above objective, a method for determining lithium plating in lithium-ion batteries based on characteristic frequency perturbation and impedance response is provided, the method comprising the following steps: Step S1: After placing the lithium-ion battery to be tested in a temperature chamber and letting it stand, start the normal charging and discharging process; Step S2: Simultaneously apply a characteristic single-frequency sinusoidal AC signal during the normal charging and discharging process to conduct a single-frequency impedance test; Step S3: Collect the real part of the impedance parameters in the single-frequency impedance test at the set intervals; Step S4: Plot the relationship between the real part of the impedance and the state of charge (SOC) based on the real part of the impedance parameters; Step S5: Identify the abrupt inflection point of the real part of the impedance in the SOC relationship diagram to complete the determination of lithium plating in the battery; Step S6: Adjust the chamber temperature and battery charging rate, and execute steps S1 to S5 in sequence. Based on the lithium plating determination results under each operating condition, draw a battery lithium plating boundary mapping diagram.

[0007] The advantages of this invention are as follows: It requires only an electrochemical workstation, charge / discharge equipment, and a temperature chamber, eliminating the need for complex devices. The testing process does not require battery disassembly, avoiding significant cell loss and resource waste, and reducing testing costs. Furthermore, by selecting a fixed 1Hz characteristic frequency within the 0.1-3Hz range and combining it with precise parameter control, the lithium plating initiation point can be accurately identified with high accuracy. It is compatible with various positive and negative electrode material systems, including lithium iron phosphate and ternary materials, covering temperatures from -10℃ to 15℃ and rate conditions from 0.3C to 1.5C, offering wide applicability. In addition, single-channel testing at a single temperature and four-rate conditions takes only 7 hours, while dual-channel parallel testing can be compressed to 3.5 hours. It can quickly generate lithium plating boundary mapping diagrams, meeting the testing requirements of mass-produced batteries, and is convenient and reliable in operation. Attached Figure Description

[0008] Figure 1 This is a schematic diagram of the steps in a lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response. Figure 2 This is a graph showing the change of the real impedance of the battery cell with SOC under charging conditions of 0°C and 0.7C in this embodiment. Figure 3 This is a graph showing the change of the real impedance of the battery cell over time at 10°C and 0.7C charging in this embodiment. Figure 4 This is a graph showing the variation of the real part impedance of the battery cell at different charging rates at 1Hz under temperatures of -10 / 0 / 10℃ in this embodiment. The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0009] The technical method of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0010] Furthermore, the accompanying drawings are merely illustrative of the invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor methods and / or microcontroller methods.

[0011] It should be understood that although the terms "first," "second," etc., may be used herein to describe various units, these units should not be limited by these terms. These terms are used merely to distinguish one unit from another. For example, without departing from the scope of the exemplary embodiments, a first unit may be referred to as a second unit, and similarly, a second unit may be referred to as a first unit. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0012] To achieve the above objectives, please refer to Figures 1 to 4 A method for determining lithium plating in lithium-ion batteries based on characteristic frequency perturbation and impedance response, the method comprising the following steps: Preferably, step S1: After placing the lithium-ion battery to be tested in a temperature chamber and letting it stand, start the normal charging and discharging process; Optionally, the lithium-ion battery to be tested in step S1 is specifically a lithium-ion battery composed of lithium iron phosphate, ternary materials and lithium-rich manganese-based positive electrode materials and graphite and silicon-based negative electrode materials.

[0013] In this embodiment, the positive electrode materials of the lithium-ion batteries under test are lithium iron phosphate, ternary materials (chemical formula LiNi0.5Co0.2Mn0.3O2), and lithium-rich manganese-based materials (chemical formula 0.3Li2MnO3・0.7LiNi0.5Mn0.5O2). The negative electrode materials are graphite and silicon-based materials (silicon-carbon composite negative electrode, silicon mass ratio 30%). Five systems of lithium-ion batteries under test are prepared according to the combination of positive and negative electrode materials. Among them, the lithium-ion battery under test with lithium iron phosphate and graphite combination is a soft pack structure with a capacity of 3.6Ah. The lithium-ion battery under test with ternary materials combined with graphite and silicon-based materials respectively is a soft pack structure with a capacity of 5Ah. The lithium-ion battery under test with lithium-rich manganese-based materials combined with graphite and silicon-based materials respectively is a soft pack structure with a capacity of 5Ah. The nominal voltage range of all lithium-ion batteries under test is 3.2V-3.7V.

[0014] Optionally, in step S1, the lithium-ion battery to be tested is placed in the temperature chamber for a period of not less than 4 hours. During the standing period, the overall temperature of the cell is kept consistent with the temperature set in the temperature chamber. The normal charge and discharge voltage range of the lithium iron phosphate soft-pack cell of the lithium-ion battery to be tested is set to 2.5-3.65V.

[0015] In this embodiment, a 3.6Ah lithium iron phosphate pouch cell is fixed inside a programmable temperature-controlled chamber using an insulating bracket, maintaining an 8cm gap between the cell and the chamber's inner wall and other components to prevent heat conduction interference. The target test temperature is input via the chamber's touch-screen control system; selectable temperatures include -10℃, 0℃, 10℃, and 15℃. The chamber's heating or cooling program is then activated. Once the ambient temperature reaches the set value, it is maintained at a constant temperature for 30 minutes, after which a timer is started for a static rest period. During the settling process, the temperature control system of the incubator continuously monitors and adjusts the internal temperature to keep the temperature fluctuation within ±0.2℃. At the same time, a platinum resistance temperature sensor with an accuracy of ±0.05℃ is attached to the center of the cell surface. The sensor is connected to an external data logger via a data transmission line. The data logger collects the cell surface temperature data every 30 seconds and simultaneously records the ambient temperature data inside the incubator. The settling time is strictly controlled to 4 hours to ensure that the difference between the cell surface temperature and the set temperature of the incubator does not exceed ±0.3℃ at the end of 4 hours, so that the overall temperature of the cell is consistent with the set temperature of the incubator.

[0016] It should be noted that after the settling period, disconnect the power supply to the temperature chamber, open the chamber door and remove the lithium iron phosphate soft-pack battery cell. Immediately connect the positive and negative terminals of the battery cell to the output terminals of the charging and discharging equipment using wires with gold-plated connectors. Use heat-shrink tubing to secure the wire connections to prevent changes in contact resistance or short circuits. Start the charging and discharging equipment and access the voltage parameter configuration interface through the equipment's host computer software. Enter 3.65V in the "Charging Cut-off Voltage" input box and 2.5V in the "Discharging Cut-off Voltage" input box. Click the "Parameter Calibration" function in the software. The equipment will automatically verify the accuracy of the input voltage parameters, ensuring that the voltage output error does not exceed ±0.005V. After successful verification, click "Parameter Save" to complete the configuration of the standard charging and discharging voltage range for the lithium iron phosphate soft-pack battery cell. During subsequent charging and discharging operations, the equipment will strictly adhere to this configured voltage range.

[0017] Preferably, step S2: a characteristic single-frequency sinusoidal AC signal is applied synchronously during the normal charging and discharging process to conduct a single-frequency impedance test; Optionally, applying a characteristic single-frequency sinusoidal AC signal synchronously during the normal charging and discharging process in step S2 includes: Wideband impedance spectrum tests were conducted on lithium-ion battery cells with different positive and negative electrode material systems. The inflection point frequency between transfer impedance and diffusion impedance was selected as the characteristic single frequency, and the value range of the characteristic single frequency was limited to 0.1-3Hz. A fixed characteristic single frequency of 1Hz is selected, and a sinusoidal AC signal of this fixed characteristic single frequency is continuously and synchronously applied during the normal charging and discharging process of the lithium-ion battery.

[0018] In this embodiment, lithium-ion battery cells with three different positive and negative electrode material systems, namely lithium iron phosphate-graphite (3.6Ah soft-pack cell), ternary material-silicon-based, and lithium-rich manganese-based-graphite, were selected from the disclosed materials. Each system cell was connected to the test port of the electrochemical workstation one by one through shielded wires. The outer layer of the wires was wrapped with a metal shielding mesh to eliminate electromagnetic interference and ensure that the positive and negative polarities were not reversed.

[0019] Each battery cell was placed in a constant temperature environment at 25℃ for 30 minutes until the cell temperature stabilized. The wide-band impedance spectroscopy test parameters of the electrochemical workstation were then set: the test frequency range was 0.01Hz-10kHz, the applied AC signal voltage amplitude was fixed at 5mV, and a linear sweep frequency mode was used, collecting 50 data points every ten octaves. After starting the test program, the workstation scanned point by point from low to high within the set frequency range, simultaneously recording the real and imaginary parts of the impedance at each frequency point. The transfer impedance and diffusion impedance values ​​at each frequency were then separated through impedance data analysis.

[0020] The transfer impedance and diffusion impedance data for each cell system were compiled and plotted as curves with the test frequency on the x-axis and the impedance value on the y-axis. The frequency corresponding to the intersection of the two curves was determined as the inflection point frequency of the transfer impedance and diffusion impedance. Testing showed that the inflection point frequencies for the three cell systems were 0.8Hz, 1.2Hz, and 2.1Hz, all falling within the 0.1-3Hz range. Based on this, the range of the characteristic single frequency was limited to 0.1-3Hz.

[0021] From the characteristic single frequency range of 0.1-3Hz, 1Hz was selected as the fixed characteristic single frequency. The signal output parameters of the electrochemical workstation were reconfigured: the signal type was set to a sinusoidal AC signal, and the output frequency was precisely calibrated to 1Hz using the workstation's frequency calibration function, with frequency fluctuations controlled within ±0.01Hz. Before the start of the regular charging and discharging process of the lithium-ion battery, the electrochemical workstation and the charging and discharging equipment were synchronized via a signal synchronization line. Simultaneously with the start of the charging and discharging equipment's charging and discharging current output, the electrochemical workstation was triggered to continuously output a 1Hz fixed characteristic single frequency sinusoidal AC signal. The signal application covered the entire charging and discharging cycle without interruption or timing delay.

[0022] Optionally, the single-frequency impedance test performed in step S2 includes: Before applying a characteristic single-frequency sinusoidal AC signal, the internal resistance of the lithium-ion battery cell under test is measured. The amplitude of the sinusoidal AC signal is determined based on the internal resistance of the battery cell, and the amplitude, after being converted into a voltage excitation signal, does not exceed 10mV. The disturbance current of the sinusoidal AC signal is fixed at 100mA. The impedance data of the battery is collected synchronously according to the sinusoidal AC signal. The acquisition process is carried out in parallel with the charging and discharging process, forming a continuous single-frequency impedance test data stream.

[0023] In this embodiment, before applying a characteristic single-frequency sinusoidal AC signal, the lithium-ion battery under test (exemplarily selected as a 3.6Ah soft-pack lithium iron phosphate-graphite cell or a 5Ah soft-pack ternary material-silicon cell) is reliably connected to the internal resistance test port of the electrochemical workstation via test leads. A constant DC current (exemplarily 100mA) is applied using the DC internal resistance test method for a continuous test duration (exemplarily 10s). The change in battery terminal voltage before and after the current application is recorded. The cell internal resistance is calculated using Ohm's law. The exemplary internal resistance of the lithium iron phosphate-graphite cell is 35mΩ, and the exemplary internal resistance of the ternary material-silicon cell is 42mΩ. The test current and test duration for the cell internal resistance can be determined according to the cell capacity, model, and performance of the test equipment to ensure the stability and accuracy of the test results.

[0024] The amplitude of the sinusoidal AC signal is determined based on the measured internal resistance of the battery cell. Following the conversion relationship of "voltage excitation signal = battery cell internal resistance × AC signal amplitude," and considering the requirement that the voltage excitation signal should not exceed 10mV, the amplitude value is calculated in reverse. For example, the amplitude of the lithium iron phosphate-graphite battery cell is set to 285.7mA, and the amplitude of the ternary material-silicon battery cell is set to 238.1mA. The converted voltage excitation signal does not exceed 10mV. In practical applications, the amplitude parameters can be flexibly adjusted according to the differences in internal resistance of different battery cell models and systems, with the core requirement of the voltage excitation signal not exceeding 10mV to ensure the reliability of the test data.

[0025] The signal output parameters of the electrochemical workstation are adjusted to a sinusoidal AC signal, with a fixed perturbation current of 100mA (this perturbation current is an example value and can be adapted and adjusted according to cell specifications, test accuracy requirements, and equipment output capabilities). Timing synchronization between the electrochemical workstation and the charge / discharge equipment is achieved via a signal synchronization line. Simultaneously with the start of the normal charge / discharge process by the charge / discharge equipment, the electrochemical workstation synchronously outputs a sinusoidal AC signal of corresponding amplitude. Impedance data from the single-frequency impedance test is acquired at exemplary intervals of 5 seconds (the acquisition interval can be adjusted to any value within the range of 1-5 seconds according to equipment capabilities and actual test requirements). The acquisition process is performed in parallel with the charge / discharge process. The acquired data includes the real part of the impedance, the imaginary part of the impedance, and the corresponding state of charge (SOC) data. All data is stored in real-time in timestamp order, forming a continuous and uninterrupted single-frequency impedance test data stream.

[0026] Preferably, step S3: Acquire the real part of the impedance parameter in the single-frequency impedance test at a set interval; Optionally, in step S3, the impedance real part parameter in the single-frequency impedance test is collected at a set interval. Specifically, the set collection interval is adjusted according to the performance of the test equipment and the actual test requirements, and the collection interval range is 1-5s.

[0027] In this embodiment, before starting the single-frequency impedance test, the performance parameters of the test equipment and the actual test requirements are first clarified: if the electrochemical workstation used has high-frequency data acquisition capability (sampling rate not less than 100Hz), and the test needs to capture the subtle change in the real part of the impedance at the beginning of lithium plating (such as the lithium plating signal under low temperature and low rate conditions), then the acquisition interval is set to 1s. The test configuration is consistent under 15℃ and 0.7C charging conditions, and the impedance change inflection point at 85% SOC can be accurately captured.

[0028] If the number of test equipment channels is limited (e.g., multiple test conditions are tested in a single channel), or if it is necessary to reduce data storage pressure and subsequent data processing volume (e.g., significant changes in lithium plating signal under high temperature and high rate conditions), then setting the acquisition interval to 5 seconds can balance test efficiency and data volume while ensuring the accuracy of lithium plating inflection point identification.

[0029] It should be noted that the acquisition interval can also be adjusted according to the characteristics of the battery cell system under test. For battery cells with different positive and negative electrode material systems such as ternary material-silicon-based and lithium-rich manganese-based-graphite, if their impedance change rate differs from that of the lithium iron phosphate-graphite system, 2s, 3s or 4s can be selected as the acquisition interval within the range of 1-5s. When adjusting, the target interval value needs to be entered through the parameter configuration interface of the electrochemical workstation, and the built-in acquisition calibration function of the device should be started to ensure that the deviation between the actual acquisition interval and the set value does not exceed ±0.1s, so as to ensure the stability and consistency of data acquisition.

[0030] Preferably, step S4: plot the relationship between the real part of the impedance and the state of charge (SOC) based on the real part of the impedance parameter; Optionally, in step S4, the relationship between the real part of the impedance and the state of charge (SOC) is plotted based on the real part of the impedance parameter. Specifically, during the plotting process, the corresponding data of the charging and discharging time and the real part of the impedance are recorded simultaneously to form a multi-dimensional data association chart.

[0031] In this embodiment, the raw data collected from the single-frequency impedance test is first organized, and the real part of the impedance parameter, the state of charge (SOC) data, and the charge / discharge time data corresponding to each acquisition are extracted in the order of timestamps. The real part of the impedance parameter is retained to six decimal places, the SOC data is accurate to 0.1%, and the charge / discharge time data is accurate to 0.01s, with the test start time as the starting point. For example, the real part of the impedance parameter of battery No. 1 (lithium iron phosphate soft pack cell, capacity 3.6Ah) collected at 15℃ and 0.7C charging conditions is 0.023568Ω, corresponding to a SOC of 12.3%.

[0032] A three-dimensional data association table was established using data processing software. The table fields were set as “charge and discharge time (s)”, “state of charge (SOC) (%)”, and “real part of impedance (Ω)”. All the processed data were entered into the table one by one according to the acquisition time sequence to ensure that the timestamp, real part of impedance, and state of charge (SOC) of each data point corresponded one-to-one without mismatch or missing data.

[0033] The system uses the State of Charge (SOC) (%) as the horizontal axis, with a range of 0%-100% and a scale interval of 5%. The real part of the impedance (Ω) is used as the vertical axis, with a range of 10% larger than the maximum and minimum values ​​of the measured real part of the impedance. For example, for the test data of battery No. 1, the range of the vertical axis is set to 0-0.03Ω and the scale interval is 0.002Ω. The data processing software's plotting function generates a two-dimensional curve showing the relationship between the real part of the impedance and the State of Charge (SOC).

[0034] While plotting the two-dimensional relationship curve, the charge and discharge time data is used as a third dimension to associate with each data point on the curve. Through the software's annotation function, clicking on any data point on the curve will display the corresponding charge and discharge time. Finally, a relationship chart containing three dimensions including the state of charge (SOC), the real part of the impedance, and the charge and discharge time is formed. The chart is saved in an editable standard vector format to ensure that the three-dimensional relationship information of any data point can be accurately extracted during subsequent data analysis.

[0035] Most importantly, when drawing multi-dimensional data correlation charts, the battery charging and discharging current, battery cell voltage, and real-time temperature of the chamber are recorded simultaneously at each data collection moment. The real part of the impedance is correlated with the state of charge (SOC), charging and discharging current, cell voltage, and ambient temperature for multi-parameter storage, forming a structured data set.

[0036] In this embodiment, the data output ports of the electrochemical workstation, charge / discharge equipment, and temperature chamber are connected to the data acquisition instrument via shielded data cables. The acquired parameters include the real part of the impedance in single-frequency impedance testing, the state of charge (SOC), the charge / discharge current, the individual cell voltage, and the real-time temperature of the temperature chamber. The acquisition frequency is consistent with the acquisition of the real part of the impedance parameters, exemplarily set at 5 seconds per acquisition as per the disclosure materials. For some operating conditions requiring precise data capture, the frequency is set to 1 second per acquisition. During data acquisition, each device outputs data synchronously: the electrochemical workstation records the real part of the impedance (with six decimal places) and the corresponding SOC (accurate to 0.1%); the charge / discharge equipment synchronously outputs the real-time charge / discharge current (in A, with three decimal places) and the individual cell voltage (in V, with four decimal places); and the temperature chamber outputs the real-time ambient temperature (in °C, with two decimal places). All data are marked with a unified timestamp (accurate to 0.01 seconds) with the test start time as the time origin. Establish a standardized data association storage format, with the field order fixed as "timestamp-real part of impedance (Ω)-state of charge (SOC) (%)-charge and discharge current (A)-cell voltage (V)-real-time temperature of the chamber (°C)". For example, the data collected at the 10th second of battery No. 1 (lithium iron phosphate soft pack cell, capacity 3.6Ah) under the charging conditions of 15°C and 0.7C (2.52A) is recorded as "10.00-0.023568-12.3-2.520-3.3256-15.02".

[0037] The multi-dimensional parameters output by each device are matched and associated one by one according to the timestamp to ensure that the parameters of each record correspond to a unique test time. The storage medium is a high-speed solid-state drive, and the data format is a common text format, which supports direct import into data analysis software. At the same time, the acquisition instrument verifies the data integrity in real time. If there is a missing parameter or a timestamp mismatch, the device will immediately trigger an abnormal prompt to ensure the accuracy and continuity of the multi-parameter association.

[0038] Preferably, step S5: identify the abrupt inflection point of the real part of the impedance in the SOC relationship diagram to complete the determination of lithium plating in the battery; Optionally, the real part of the impedance consists of the ohmic impedance R0, the film impedance Rsei, and the charge transfer impedance Rct. When lithium plating occurs at the negative electrode of the battery, the charge transfer impedance Rct decreases, and the real part of the impedance shows an abrupt inflection point, which corresponds to the starting point of the lithium plating reaction.

[0039] In this embodiment, before identifying the inflection point of the abrupt change in the real part of the impedance, the composition of the real part of the impedance at the characteristic frequency of 1Hz is defined, and its value is the superposition of the ohmic impedance R0, the membrane impedance Rsei, and the charge transfer impedance Rct. Taking a No. 1 battery (lithium iron phosphate soft-pack cell, capacity 3.6Ah) as the test object, under the charging conditions of 15℃ and 0.7C (2.52A), the initial real part data of the impedance collected by the electrochemical workstation is 0.0235Ω, which includes the ohmic impedance R0=0.0052Ω, the membrane impedance Rsei=0.0103Ω, and the charge transfer impedance Rct=0.008Ω. The superposition of the three values ​​is consistent with the measured real part value of the impedance.

[0040] During the simultaneous routine charging and discharging of lithium-ion batteries and single-frequency impedance testing, the real part of the impedance and the corresponding state of charge (SOC) data are continuously recorded at set acquisition intervals. For example, during the process of SOC increasing from 0% to 85% for battery No. 1, the real part of the impedance remains stable in the range of 0.022Ω-0.0235Ω, and the charge transfer impedance Rct remains in the range of 0.0075Ω-0.008Ω without significant fluctuations.

[0041] When the State of Charge (SOC) of Battery 1 reaches 85%, the real part of the impedance value suddenly changes to 0.0198Ω, a decrease of 0.0032Ω compared to the previous acquisition time. Through impedance composition decomposition, it can be seen that the values ​​of ohmic impedance R0 and film impedance Rsei have not changed, while the charge transfer impedance Rct drops to 0.0043Ω, a decrease of 46.25%. Therefore, the sudden change point of the real part of the impedance is determined to be the starting point of the lithium plating reaction, and the corresponding SOC of 85% is the critical state of lithium plating under this condition.

[0042] For other battery cells under test, such as ternary material-silicon-based and lithium-rich manganese-based-graphite, the real part of the impedance is recorded as a function of the state of charge (SOC) during charge and discharge, following the same procedure. When a sudden drop in the real part of the impedance is detected, and the drop is confirmed to be due to a decrease in the charge transfer impedance Rct after decomposition, the SOC corresponding to that moment is immediately locked as the starting point of the lithium plating reaction of the battery cell under the current test conditions.

[0043] Most importantly, when identifying the abrupt change in the real part of the impedance, a threshold for determining the abrupt change is set. When the cumulative decrease in the real part of the impedance value collected in three consecutive tests exceeds 10% of the initial real part of the impedance value, it is determined that an abrupt change in the inflection point has occurred. At the same time, the state of charge (SOC), charge and discharge time, and current operating parameters corresponding to the inflection point are recorded.

[0044] In this embodiment, before identifying the abrupt change in the real part of the impedance, the initial standard for calculating the real part of the impedance is first determined: the real part data of the impedance collected in the first 10 tests after the start of the single-frequency impedance test are selected, and the initial benchmark value is calculated by the arithmetic mean method. For example, for battery No. 1 (lithium iron phosphate soft pack cell, capacity 3.6Ah), under the charging conditions of 15℃ and 0.7C (2.52A), the real part data of the impedance collected in the first 10 tests are 0.0235Ω, 0.0236Ω, 0.0234Ω, 0.0235Ω, 0.0237Ω, 0.0235Ω, 0.0236Ω, 0.0234Ω, 0.0235Ω, and 0.0236Ω, respectively. The average value of the real part of the initial impedance is calculated to be 0.0235Ω, which is used as the benchmark for determining the abrupt change.

[0045] The threshold for determining a sudden change is set to 10% of the real part of the initial impedance. For example, the threshold for battery number 1 is 0.00235Ω. This threshold applies to all test conditions and is adjusted synchronously only with changes in the reference value of the real part of the initial impedance. During the simultaneous charging / discharging of the lithium-ion battery and the single-frequency impedance test, impedance real part data is continuously acquired at a set acquisition interval (1 second for example). Each new data acquisition is combined with the previous two acquisitions to form three consecutive sets of data. The decrease in the value of the last two data points relative to the previous data point is calculated, and the total decrease between the three data points is accumulated.

[0046] When the State of Charge (SOC) of battery No. 1 reaches 83%, the real part of the impedance measured in the nth sampling is 0.0234Ω, in the (n+1)th sampling it is 0.0225Ω, and in the (n+2)th sampling it is 0.0210Ω. The cumulative decrease of these three data points is 0.0024Ω, exceeding the threshold of 0.00235Ω, thus immediately identifying a sudden inflection point. Simultaneously, the SOC corresponding to this inflection point is locked at 85%, the charging / discharging time at 3200s, and the current operating parameters are recorded: real-time chamber temperature 15.0℃, charging rate 0.7C, charging current 2.52A, and battery cell voltage 3.48V. All parameters are stored with timestamps to ensure a one-to-one correspondence with the sudden inflection point.

[0047] For other battery cell systems such as ternary material-silicon-based and lithium-rich manganese-graphite-based, or for different temperature conditions such as -10℃, 0℃, and 10℃, and for different charging rates such as 0.3C, 0.5C, 1C, and 1.5C, the same process is used to calculate the initial impedance real part reference value, set a 10% threshold for sudden change judgment, and calculate the decrease in data collected three times consecutively. When the cumulative decrease exceeds the threshold, it is judged as a sudden change inflection point and the corresponding parameters are fully recorded to ensure the consistency of judgment standards and the integrity of data under different systems and different operating conditions.

[0048] Preferably, in step S6: adjust the temperature of the incubator and the battery charging rate, and execute steps S1 to S5 in sequence. Based on the lithium plating determination results under each operating condition, draw a battery lithium plating boundary mapping diagram.

[0049] Optionally, adjusting the chamber temperature and battery charging rate in step S6 specifically involves: The adjusted chamber temperatures should cover at least -10℃, 0℃, 10℃, and 15℃; The adjusted charging rates cover at least 0.3C, 0.5C, 0.7C, 1C, and 1.5C; All combinations of chamber temperature and charging rate are executed using steps S1 to S5.

[0050] In this embodiment, the temperature adjustment configuration of the temperature chamber is first completed. The target temperatures of the programmable temperature control chamber are set to -10℃, 0℃, 10℃, and 15℃, respectively. After each temperature setting, the temperature control program of the temperature chamber is started. After the temperature inside the chamber reaches the set value and stabilizes for 30 minutes, the cell under test is placed in a static state to ensure the stability of the test temperature, with temperature fluctuations controlled within ±0.5℃. Simultaneously, the charging rate adjustment configuration is completed. For each set temperature, the charging rate of the charging and discharging equipment is adjusted to 0.3C, 0.5C, 0.7C, 1C, and 1.5C, respectively. The corresponding charging current is calculated according to the capacity of the cell under test: taking a 3.6Ah lithium iron phosphate soft-pack cell as an example, 0.3C corresponds to a charging current of 1.08A, 0.5C corresponds to 1.8A, 0.7C corresponds to 2.52A, 1C corresponds to 3.6A, and 1.5C corresponds to 5.4A. After the charging rate is adjusted, the current output accuracy error is ensured to be no more than ±0.01A through the equipment calibration function.

[0051] A complete list of test conditions is generated based on the combination of chamber temperature and charging rate, resulting in 20 sets of combined conditions (4 temperatures × 5 rates). Exemplary conditions include -10℃ +0.3C, 0℃ +0.7C, 10℃ +1C, and 15℃ +1.5C. For each set of combined conditions, steps S1 to S5 are executed sequentially: the cell under test is placed in the corresponding temperature chamber and left to stand for at least 4 hours; the charging and discharging voltage range is set; a 1Hz characteristic single-frequency sinusoidal AC signal is applied simultaneously to conduct single-frequency impedance testing; the real part of the impedance parameter is collected at set intervals; the relationship between the real part of the impedance and SOC is plotted, and the abrupt inflection point is identified; the SOC and test condition parameters corresponding to the lithium plating initiation point are recorded.

[0052] After all combined operating condition tests are completed, the lithium plating initiation point data for each group of operating conditions are summarized to ensure that each group of data includes key parameters such as chamber temperature, charging rate, critical state of lithium plating (SOC), and test time. The data recording format is uniformly "temperature-rate-critical state of lithium plating (SOC)," with an example record of "15℃-0.7C-85%SOC," providing a complete and standardized data source for subsequent drawing of battery lithium plating boundary mapping diagrams.

[0053] Preferably, the equipment used in the testing process of steps S1 to S5 includes: an electrochemical workstation, a charge-discharge device, and a temperature chamber; the electrochemical workstation is used to apply a sinusoidal AC signal and collect impedance data, the charge-discharge device is used to perform a conventional charge-discharge process, and the temperature chamber is used to regulate the test environment temperature; under a single temperature condition, when testing is performed at four rates of 0.3C, 0.5C, 1C, and 1.5C; the single-channel test duration does not exceed 7 hours, and when using dual-channel parallel testing, the test duration is controlled within 3.5 hours.

[0054] In this embodiment, an electrochemical workstation with single-frequency signal output and impedance data acquisition functions (supporting a frequency range of 0.01Hz-10kHz, voltage output accuracy ±0.001V), a programmable charge-discharge device (current output range 0-10A, voltage control accuracy ±0.01V), and a programmable temperature-controlled chamber (temperature control range -40℃ to 85℃, temperature fluctuation ±0.5℃) are selected. These three components are connected via shielded data cables and signal synchronization lines to ensure timing consistency and data transmission stability between devices. The electrochemical workstation is functionally positioned to apply a characteristic single-frequency sinusoidal AC signal and acquire impedance data: a sinusoidal AC signal with an output frequency of 1Hz and a disturbance current of 100mA is set according to the disclosed materials. The signal amplitude is calculated based on the internal resistance of the cell under test, ensuring that the voltage excitation signal does not exceed 10mV. Simultaneously, the real and imaginary parts of the impedance are acquired at set intervals (exemplarily 1s or 5s), with a data sampling rate of not less than 100Hz, retaining six decimal places of precision. The charging and discharging equipment is responsible for executing the conventional charging and discharging process, outputting a constant current according to the set charging rate, and simultaneously recording the State of Charge (SOC), individual cell voltage data, and synchronously correlated with impedance data using timestamps. The temperature chamber is used to regulate the test environment temperature, stabilizing it at -10℃, 0℃, 10℃, or 15℃ according to operating conditions, ensuring the overall cell temperature is consistent with the ambient temperature. For a single temperature condition, four charging rates—0.3C, 0.5C, 1C, and 1.5C—are selected for testing: taking a 3.6Ah lithium iron phosphate soft-pack cell as an example, the corresponding charging currents for each rate are 1.08A, 1.8A, 3.6A, and 5.4A, respectively. When using single-channel testing, steps S1 to S5 are executed sequentially according to the charging rate, with each rate test lasting approximately 1.75 hours, and the total test time for the four rates not exceeding 7 hours. If a dual-channel charge-discharge device and a dual-interface electrochemical workstation are used for parallel testing, the two sets of cells under test are connected to two test channels respectively, and the test process at different rates is started simultaneously. For example, channel 1 performs 0.3C and 1C rate tests, and channel 2 performs 0.5C and 1.5C rate tests. The tests of each channel are carried out independently and the data do not interfere with each other. The overall test time is controlled within 3.5 hours, achieving a doubling of test efficiency.

[0055] This invention significantly shortens the overall testing time and reduces testing costs by conducting tests under different temperature conditions and with different charging rates. If four charging rates of 0.3C, 0.5C, 1C, and 1.5C are selected for testing under a single temperature condition, the total testing time for a single temperature is approximately 7 hours when using a single-channel testing mode; however, if a dual-channel parallel testing mode is used, the testing time for a single temperature can be reduced to less than 3.5 hours.

[0056] The present invention also provides the following embodiments: Implementation Case 1: A No. 1 battery (lithium iron phosphate soft-pack cell, capacity 3.6Ah) was selected as the test object. It was placed in a temperature chamber for at least 4 hours to ensure the overall cell temperature stabilized at 15℃. The normal charge / discharge voltage range of the cell was set to 2.5V-3.65V using a charge / discharge device, and the charging current was set to 2.52A (corresponding to a 0.7C charging rate). Simultaneously, a sinusoidal AC signal with a characteristic frequency of 1Hz was applied using an electrochemical workstation, with a disturbance current of 100mA, and data was continuously collected at 1-second intervals. Analysis of the collected data revealed a significant abrupt change in the real part of the impedance when the State of Charge (SOC) reached 85%. Specifically, the curve exhibited a precipitous drop at this SOC node, a stark contrast to the previous stable trend. For detailed data changes, please refer to [link to relevant documentation]. Figure 1 The graph shows the change of real impedance with SOC under charging conditions of 0℃ and 0.7C. Battery No. 2 was also charged under the same conditions. Charging was stopped before the inflection point of the real impedance abruptly appeared. The corresponding real impedance curve remained stable throughout, without significant fluctuations or a downward inflection point, showing a slow and gradual trend. For detailed data changes, please refer to [reference needed]. Figure 2 The graph shows the change of the real impedance over time under charging conditions of 10℃ and 0.7C. Implementation Case 2: By testing the battery cell under different temperature conditions such as -10℃, 0℃, and 10℃ with different charging rates, the lithium deposition boundary of the battery cell under each temperature condition was plotted. The specific test results are summarized in Table 1.

[0057] Table 1 Table 1 above presents the maximum safe charging rate of the battery cell in different SOC ranges (0-10%, 10-20%, ..., 90-100%) under three temperature conditions: -10℃, 0℃, and 10℃. It intuitively reflects the influence of temperature and SOC on the lithium plating boundary of the battery cell. In the -10℃ low-temperature condition, the battery cell can only withstand a 0.3C charging rate in the 0-10% SOC range. After entering the 10% SOC range, the maximum safe charging rate drops to 0.15C and remains at this low rate throughout, indicating that low temperature significantly limits the charging capability of the battery cell. In the 0℃ condition, the maximum safe charging rate in the 0-20% SOC range is 0.7C, which drops to 0.45C after 20% SOC, and further to 0.2C after 80% SOC, showing a trend of gradually decreasing with increasing SOC. In the 10℃ condition, the battery cell can stably withstand a high-rate charging of 1.5C in the 0-70% SOC range. After 70% SOC, the maximum safe charging rate drops to 1.0C, demonstrating that the battery cell has a stronger high-rate charging capability at higher temperatures. This table systematically summarizes the safe charging boundaries of the battery cell under different operating conditions, providing direct data support for selecting a reasonable charging rate based on temperature and SOC in practical applications and avoiding lithium plating.

[0058] Please see Figure 4 The invention presents the 1Hz real impedance versus SOC curves at different charging rates (e.g., 0.2C, 0.3C, 0.5C, 0.7C) under operating conditions of -10 / 0 / 10℃. The curves clearly show the variation of the real impedance under different operating conditions and the SOC nodes where abrupt inflection points occur, intuitively reflecting the influence of temperature and charging rate on the lithium plating initiation point. This invention eliminates the need for extensive cell disassembly, avoiding waste of testing resources. It can quickly and accurately identify the lithium plating boundary of the cell and can be flexibly tested according to adjustments based on actual operating conditions. It has stronger applicability for lithium plating detection in mass-produced batteries, and the overall operation process is simple, convenient, and reliable.

[0059] Therefore, the embodiments should be considered as exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the application are intended to be included within the invention.

[0060] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features of the invention herein.

Claims

1. A method for determining lithium plating in lithium-ion batteries based on characteristic frequency perturbation and impedance response, characterized in that, Includes the following steps: Step S1: After placing the lithium-ion battery to be tested in a temperature chamber and letting it stand, start the normal charging and discharging process; Step S2: Simultaneously apply a characteristic single-frequency sinusoidal AC signal during the normal charging and discharging process to conduct a single-frequency impedance test; Step S3: Collect the real part of the impedance parameters in the single-frequency impedance test at the set intervals; Step S4: Plot the relationship between the real part of the impedance and the state of charge (SOC) based on the real part of the impedance parameters; Step S5: Identify the abrupt inflection point of the real part of the impedance in the SOC relationship diagram to complete the determination of lithium plating in the battery; Step S6: Adjust the chamber temperature and battery charging rate, and execute steps S1 to S5 in sequence. Based on the lithium plating determination results under each operating condition, draw a battery lithium plating boundary mapping diagram.

2. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, The lithium-ion battery to be tested in step S1 is specifically a lithium-ion battery composed of lithium iron phosphate, ternary materials and lithium-rich manganese-based positive electrode materials and graphite and silicon-based negative electrode materials.

3. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, In step S1, the lithium-ion battery to be tested is placed in a temperature chamber for a period of not less than 4 hours. During the resting process, the overall temperature of the cell is kept consistent with the temperature set in the temperature chamber. The normal charge and discharge voltage range of the lithium iron phosphate soft-pack cell of the lithium-ion battery to be tested is set to 2.5-3.65V.

4. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, Step S2, which involves synchronously applying a characteristic single-frequency sinusoidal AC signal during the normal charging and discharging process, includes: Wideband impedance spectrum tests were conducted on lithium-ion battery cells with different positive and negative electrode material systems. The inflection point frequency between transfer impedance and diffusion impedance was selected as the characteristic single frequency, and the value range of the characteristic single frequency was limited to 0.1-3Hz. A fixed characteristic single frequency of 1Hz is selected, and a sinusoidal AC signal of this fixed characteristic single frequency is continuously and synchronously applied during the normal charging and discharging process of the lithium-ion battery.

5. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, Step S2, which involves performing a single-frequency impedance test, includes: Before applying a characteristic single-frequency sinusoidal AC signal, the internal resistance of the lithium-ion battery cell under test is measured. The amplitude of the sinusoidal AC signal is determined based on the internal resistance of the battery cell, and the amplitude, after being converted into a voltage excitation signal, does not exceed 10mV. The disturbance current of the sinusoidal AC signal is fixed at 100mA. The impedance data of the battery is collected synchronously according to the sinusoidal AC signal. The acquisition process is carried out in parallel with the charging and discharging process, forming a continuous single-frequency impedance test data stream.

6. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, In step S3, the impedance real part parameter in the single-frequency impedance test is collected at a set interval. Specifically, the set collection interval is adjusted according to the performance of the test equipment and the actual test requirements. The collection interval range is 1-5s.

7. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, In step S4, the relationship between the real part of the impedance and the state of charge (SOC) is plotted based on the real part of the impedance parameter. Specifically, during the plotting process, the corresponding data of the charging and discharging time and the real part of the impedance are recorded simultaneously to form a multi-dimensional data correlation chart.

8. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, In step S5, identifying the abrupt inflection point of the real part of the impedance in the SOC relationship diagram is specifically as follows: The real part of the impedance consists of the ohmic impedance R0, the film impedance Rsei, and the charge transfer impedance Rct. When lithium plating occurs at the negative electrode of the battery, the charge transfer impedance Rct decreases, and the real part of the impedance shows an abrupt inflection point, which corresponds to the starting point of the lithium plating reaction.

9. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, In step S6, adjusting the chamber temperature and battery charging rate specifically involves: The adjusted chamber temperatures should cover at least -10℃, 0℃, 10℃, and 15℃; The adjusted charging rates cover at least 0.3C, 0.5C, 0.7C, 1C, and 1.5C; All combinations of chamber temperature and charging rate are executed using steps S1 to S5.

10. The lithium-ion battery lithium plating determination method based on characteristic frequency perturbation and impedance response according to claim 1, characterized in that, The equipment used in the testing process from steps S1 to S5 is specifically: an electrochemical workstation, a charge-discharge device, and a temperature chamber; the electrochemical workstation is used to apply a sinusoidal AC signal and collect impedance data, the charge-discharge device is used to perform the conventional charge-discharge process, and the temperature chamber is used to regulate the test environment temperature; under a single temperature condition, when testing is performed at four rates of 0.3C, 0.5C, 1C, and 1.5C; the single-channel test duration shall not exceed 7 hours, and when using dual-channel parallel testing, the test duration shall be controlled within 3.5 hours.

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