Lithium precipitation discrimination method for lithium ion battery based on nonlinear alternating current response characteristics

Abstract

The application provides a lithium ion battery lithium precipitation discrimination method based on a nonlinear alternating current response characteristic, and comprises the following steps: step 1, during the whole charging process of the lithium ion battery, an alternating current excitation signal at a preset frequency is applied to the lithium ion battery, and corresponding voltage signals at different states of charge (SOC) are obtained; step 2, a nonlinear alternating current response characteristic-SOC curve is obtained; step 3, all extreme points of the nonlinear alternating current response characteristic-SOC curve are identified; step 4, an inflection point is determined: all points on the battery negative electrode potential SOC curve are traversed, and a point with a negative electrode potential of 0 is found, which is recorded as point A; all extreme points are traversed, and an extreme point with a SOC close to that of point A is found, which is taken as the inflection point; and step 5, lithium precipitation discrimination: the inflection point is taken as the starting point of lithium precipitation. The application realizes accurate identification of the lithium precipitation behavior by using the second harmonic change caused by the enhanced nonlinear characteristic of the battery during the lithium precipitation process, and does not need to rely on battery disassembly or implantation of a reference electrode.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery state monitoring and safety assessment technology, and particularly relates to a lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics. Background Technology

[0002] Lithium-ion batteries are widely used in new energy vehicles, energy storage systems, and portable electronic devices due to their high energy density and long cycle life. With the development of fast-charging technology, lithium-ion batteries are prone to lithium plating at the negative electrode under conditions such as high-rate charging, low-temperature environments, or battery aging. Lithium plating leads to battery capacity decay, increased internal resistance, and in severe cases, may even cause internal short circuits and safety accidents. Therefore, accurate identification of lithium plating behavior in lithium-ion batteries is of great significance.

[0003] In existing technologies, the methods for identifying lithium plating in lithium-ion batteries mainly include the following categories: One type of method is based on the analysis of macroscopic electrical parameters such as voltage and current. For example, it can determine the occurrence of lithium plating by the change of charging voltage plateau, relaxation voltage characteristics or coulombic efficiency. However, this type of method cannot accurately determine the start time of lithium plating, but can only determine whether lithium plating has occurred under the current operating conditions and usage conditions. Another type of method is based on temperature or thermal property changes for analysis, but this type of method is usually not sensitive to the early stages of lithium plating and is easily affected by ambient temperature. Some methods obtain negative electrode state information by setting a negative electrode reference electrode or by disassembling and analyzing the battery. These methods are destructive analyses and are difficult to implement in engineering applications.

[0004] In addition, some technologies have attempted to analyze the state of lithium-ion batteries using AC impedance spectroscopy, mainly focusing on indirectly reflecting the internal state of the battery based on changes in linear AC impedance characteristics (such as the real part, imaginary part of impedance, or equivalent circuit model parameters). However, lithium plating is a typical strongly nonlinear electrochemical process, and linear AC impedance analysis methods are difficult to fully characterize this type of nonlinear behavior. Furthermore, they usually require measurements at multiple frequency points and fitting with complex equivalent models, resulting in unclear linear characteristics, complex implementation processes, and limitations in real-time performance and engineering applicability.

[0005] In recent years, nonlinear AC impedance analysis has gradually attracted attention. This type of method reflects the nonlinear characteristics of the internal electrochemical processes by analyzing the high-order harmonic response generated by the battery under nonlinear excitation conditions. However, existing related studies mostly focus on nonlinear mechanism analysis or discussion of experimental phenomena, and have not yet formed a lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics, especially based on second harmonic characteristics. Furthermore, the correspondence between nonlinear harmonic characteristics and the lithiation phase transition and lithium plating behavior of graphite anodes has not been clearly established, making it difficult to meet the need for online and accurate lithium plating discrimination of lithium-ion batteries under high-risk operating conditions such as fast charging. Summary of the Invention

[0006] The purpose of this invention is to provide a method for identifying lithium plating in lithium-ion batteries based on nonlinear AC response characteristics. This method extracts the nonlinear frequency response characteristics of the battery under sinusoidal signal excitation and establishes a correspondence between these characteristics and the lithiation phase transition behavior of the graphite anode (response to the anode potential), thereby achieving accurate identification of lithium plating in lithium-ion batteries. The technical solution adopted is as follows: A lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics includes the following steps: Step 1: During the entire charging process of the lithium-ion battery, apply a preset frequency to the lithium-ion battery. The AC excitation signal is used to obtain the voltage signal corresponding to different states of charge (SOC). The SOC computing scheme is an existing technology.

[0007] Step 2: Obtain the nonlinear AC response characteristics - SOC curve: Frequency domain analysis of the voltage signal is performed to obtain the nonlinear AC response characteristics under each state of charge (SOC). All states of charge (SOC) and all nonlinear AC response characteristics form the nonlinear AC response characteristic-SOC curve; Step 3: Identify all extreme points of the nonlinear AC response characteristic - the SOC curve; Wherein, extreme point = (nonlinear AC response characteristics, SOC); Step 4: Determine the inflection point: Traverse all points on the battery negative electrode potential-SOC curve, find the point where the negative electrode potential is 0, and denote it as point A; point A = (0, SOC) / (0, SOC). A The negative electrode potential-SOC curve of the battery was obtained through preliminary experiments. Iterate through all extreme points and find the point where A = (0, SOC) A The extreme points within the SOC range of ) are taken as inflection points; The SOC range is: [SOC] A -δ, SOC A +δ]; δ not exceeding 10%; Step 5, Lithium plating detection: The inflection point is the starting point of lithium plating.

[0008] Preferably, the nonlinear AC response characteristic is the amplitude of the second harmonic component.

[0009] Preferably, step 2 specifically includes the following steps: The voltage signal is analyzed in the frequency domain using the Fourier transform method to obtain the fundamental frequency component and the Nth harmonic component; N≥2; The amplitude of the second harmonic component is selected as the characteristic of the nonlinear AC response.

[0010] Preferably, obtaining the battery negative electrode potential-SOC curve specifically includes the following steps: Step A: Select and disassemble lithium-ion batteries of the same specifications as the lithium-ion batteries to be tested, and prepare negative electrode pairs for lithium half-cells. Step B: During the entire discharge process of the lithium half-cell, acquire the negative electrode potential signal corresponding to different states of charge (SOC) of the lithium-ion battery; wherein, the SOC calculation scheme is existing technology.

[0011] All states of charge (SOC) and all negative electrode potentials form the battery negative electrode potential-SOC curve.

[0012] Preferably, the extreme point is obtained by using an extreme value search algorithm or by calculating the first derivative.

[0013] Preferably, the AC excitation signal is a sinusoidal current or sinusoidal voltage signal with a preset frequency. <10Hz.

[0014] Compared with the prior art, the advantages of the present invention are: First, we propose using the inflection point of the nonlinear AC response characteristics as the criterion for lithium plating. Compared with existing technologies, this method uses only a fixed frequency for measuring impedance and does not require fitting equivalent models to impedances at different frequencies, which has the advantage of high efficiency.

[0015] Second, it does not require setting a reference electrode or disassembling the battery, and has the characteristics of in-situ non-destructive identification and good engineering feasibility. It can be integrated into the battery management system to realize online monitoring and application.

[0016] Third, it does not rely on complex offline calibration work, reducing the workload of offline calibration. It has high sensitivity to early lithium plating and is suitable for high-risk conditions such as fast charging. Attached Figure Description

[0017] Figure 1 This is a flowchart of a lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics.

[0018] Figure 2 This is a graph showing the variation of the nonlinear AC response characteristics of a lithium-ion battery under different charging currents.

[0019] Figure 3 This is a graph showing the variation of the nonlinear AC response characteristics of a lithium-ion battery under different AC excitation signal amplitudes.

[0020] Figure 4 This is a diagram illustrating the principle of inflection point determination.

[0021] Figure 5 This is a graph showing the characteristic changes of the second harmonic amplitude Y2 and the real part of the fundamental frequency AC impedance during the charging process. Detailed Implementation

[0022] The lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics of the present invention will be described in more detail below with reference to the schematic diagrams, which illustrate preferred embodiments of the present invention. It should be understood that those skilled in the art can modify the present invention described herein while still achieving the advantageous effects of the present invention. Therefore, the following description should be understood as being of general knowledge to those skilled in the art and is not intended to limit the present invention.

[0023] like Figure 1 As shown, a lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics includes the following steps: Step 1: During the entire charging process of the lithium-ion battery, apply a preset frequency to the lithium-ion battery. The AC excitation signal is used to obtain the voltage signal corresponding to different states of charge (SOC).

[0024] The AC excitation signal, i.e., the AC excitation current, is a sinusoidal current signal with a preset frequency. =1Hz, the current magnitude corresponding to the AC excitation signal amplitude is equivalent to 0.7C of the DC charging rate, hereinafter referred to as the excitation rate.

[0025] The DC charging rate is 1C. The excitation rate is 0.7C.

[0026] To ensure that no extreme points are missed in the nonlinear AC response characteristic-SOC curve in step 2, an AC excitation signal is applied throughout the entire charging process.

[0027] Step 2: Obtain the nonlinear AC response characteristics - SOC curve.

[0028] All states of charge (SOC) and all nonlinear AC response characteristics form the nonlinear AC response characteristic-SOC curve.

[0029] Frequency domain analysis of the voltage signal is performed to obtain the nonlinear AC response characteristics under each state of charge (SOC). Specifically, this includes: The voltage signal is analyzed in the frequency domain using the Fourier transform method to obtain the fundamental frequency component and the Nth harmonic component; N≥2; The amplitude of the second harmonic component is selected as the characteristic of the nonlinear AC response.

[0030] The second harmonic has the largest response component among all higher harmonics, so the signal-to-noise ratio obtained from its acquisition and calculation is the best (with the smallest noise spikes), making it easier to analyze.

[0031] The amplitude of the second harmonic component, abbreviated as second harmonic amplitude Y2.

[0032] Figure 4 The first curve in the diagram is the nonlinear AC response characteristic curve – the SOC curve.

[0033] Step 3: Identify the extreme points of the nonlinear AC response characteristic - the SOC curve.

[0034] Where, the extreme point = (amplitude of the second harmonic component, SOC).

[0035] Extreme points can be identified by using an extreme value search algorithm or by taking the first derivative.

[0036] Step 4: Determine the inflection point.

[0037] Traverse all points on the battery negative electrode potential-SOC curve, find the point where the negative electrode potential is 0, and denote it as point A; point A = (0, SOC) / (0, SOC). A The negative electrode potential-SOC curve of the battery was obtained through preliminary experiments. Iterate through all extreme points and find the point where A = (0, SOC) A The SOC range near [SOC] A -δ, SOC A The extreme point of +δ] is taken as the inflection point; the δ in the SOC range does not exceed 10%; In this embodiment, the inflection point is... Figure 4 The intersection of the vertical dashed line and the nonlinear AC response characteristic - SOC.

[0038] According to existing technology, lithium deposition begins when the negative electrode potential of a lithium-ion battery drops to 0V.

[0039] The acquisition of the battery negative electrode potential-SOC curve specifically includes the following steps: Step A: Select and disassemble a lithium-ion battery of the same specifications as the lithium-ion battery tested in Step 1 to prepare a negative electrode to a lithium half-cell. The negative electrode is the working electrode material of the lithium half-cell, the counter electrode is a lithium metal sheet, and the electrolyte and separator are materials with the same composition collected from the disassembled lithium-ion battery under test. Step B: During the entire discharge process of the negative electrode to the lithium half-cell, acquire the negative electrode potential signal corresponding to different states of charge (SOC) of the equivalent lithium-ion battery under test. All states of charge (SOC) and all negative electrode potentials form the battery negative electrode potential-SOC curve.

[0040] Figure 4 The second curve in the figure is the battery negative electrode potential-SOC curve.

[0041] Step 5: Lithium plating detection.

[0042] The inflection point is taken as the starting point of lithium plating.

[0043] like Figure 4 As shown, the variation trend of the second harmonic amplitude Y2 with SOC exhibits multiple peaks and extreme points, and this variation trend is consistent with the evolution law of the graphite negative electrode potential plateau.

[0044] Furthermore, when the last peak appears in the nonlinear AC response characteristic - SOC curve, the corresponding graphite negative electrode potential of the battery is below 0V, indicating that lithium plating has occurred on the negative electrode surface.

[0045] The implementation principle of this invention is as follows: During the charging process of a lithium-ion battery, lithium ions are gradually embedded into the graphite material of the negative electrode. As the degree of graphitization increases, the negative electrode undergoes multiple lithiation phase transition stages, which are reflected as different potential plateaus in the negative electrode potential (voltage) curve. During the phase transition, the electrode reaction kinetics and interface states change, significantly enhancing the nonlinearity of the battery system.

[0046] Due to the nonlinear characteristics of the battery, when an AC excitation signal is superimposed, higher harmonic components will be generated in the battery response. These higher harmonic components are the Nth harmonic components.

[0047] In this embodiment, while a constant current charging of an 18650 type ternary lithium-ion battery with a rated capacity of 2.75Ah is performed, an AC excitation signal is applied to obtain the following: Figure 2 The figure shows the variation of the second harmonic amplitude Y2 under different charging rates.

[0048] Figure 2 The "current ratio" refers to the DC charging ratio.

[0049] Figure 2This indicates that under higher DC charging rates, the second harmonic amplitude Y2 will reach a new inflection point during the charging process, i.e., lithium plating will occur in the battery.

[0050] Figure 3 The "excitation rate" in this context is a quantity used to describe the amplitude of the AC excitation current, similar to the expression for DC charging rate. Excitation rate = AC excitation current / battery rated capacity.

[0051] Figure 3 It can be seen that the inflection point is mainly caused by the change in DC charging current (change in DC charging rate), while the change in AC excitation amplitude has little effect on the position of the inflection point and the corresponding SOC.

[0052] like Figure 5 As shown in the figure below, at the same time as the last peak appears after the inflection point, the real part of the fundamental frequency AC impedance shows a clear accelerating decreasing trend, which further indicates that the inflection point appearing in the nonlinear AC response characteristic-SOC curve can be used as the criterion for judging the occurrence of lithium plating in lithium-ion batteries.

[0053] Therefore, during the lithium-ion battery charging process, by real-time monitoring of the second harmonic amplitude characteristics of the nonlinear AC impedance, when the linear AC response characteristic - SOC curve is detected in the characteristic range [SOC], A -δ, SOC A When +δ] reaches an extreme point, it can be determined that lithium plating has occurred in the battery under the current charging conditions.

[0054] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the present invention. Any equivalent substitutions or modifications made by those skilled in the art to the technical solutions and content disclosed in the present invention without departing from the scope of the present invention shall be deemed to have remained within the protection scope of the present invention.

Claims

1. A method for identifying lithium plating in lithium-ion batteries based on nonlinear AC response characteristics, characterized in that, Includes the following steps: Step 1: During the entire charging process of the lithium-ion battery, apply a preset frequency to the lithium-ion battery. The AC excitation signal is used to obtain the voltage signal corresponding to different states of charge (SOC). Step 2: Obtain the nonlinear AC response characteristics - SOC curve: Frequency domain analysis of the voltage signal is performed to obtain the nonlinear AC response characteristics under each state of charge (SOC). All states of charge (SOC) and all nonlinear AC response characteristics form the nonlinear AC response characteristic-SOC curve; Step 3: Identify all extreme points of the nonlinear AC response characteristic - the SOC curve; Wherein, extreme point = (nonlinear AC response characteristics, SOC); Step 4: Determine the inflection point: Traverse all points on the battery negative electrode potential-SOC curve, find the point where the negative electrode potential is 0, and denote it as point A; point A = (0, SOC) / (0, SOC). A The negative electrode potential-SOC curve of the battery was obtained through preliminary experiments. Iterate through all extreme points and find the point where A = (0, SOC) A The extreme points within the SOC range of ) are taken as inflection points; The SOC range is: [SOC] A -δ, SOC A +δ]; δ not exceeding 10%; Step 5, Lithium plating detection: The inflection point is the starting point of lithium plating.

2. The lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics according to claim 1, characterized in that, The nonlinear AC response characteristic is the amplitude of the second harmonic component.

3. The lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics according to claim 2, characterized in that, Step 2 specifically includes the following steps: The voltage signal is analyzed in the frequency domain using the Fourier transform method to obtain the fundamental frequency component and the Nth harmonic component; N≥2; The amplitude of the second harmonic component is selected as the characteristic of the nonlinear AC response.

4. The lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics according to claim 1, characterized in that, Obtaining the battery negative electrode potential-SOC curve includes the following steps: Step A: Select and disassemble lithium-ion batteries of the same specifications as the lithium-ion batteries being tested to prepare a negative electrode to a lithium half-cell. Step B: During the entire discharge process of the lithium half-cell, acquire the negative electrode potential signal corresponding to different states of charge (SOC) of the lithium-ion battery. All states of charge (SOC) and all negative electrode potentials form the battery negative electrode potential-SOC curve.

5. The lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics according to claim 1, characterized in that, The extreme points are obtained by using an extreme value search algorithm or by taking the first derivative.

6. The lithium-ion battery lithium plating discrimination method based on nonlinear AC response characteristics according to claim 1, characterized in that, The AC excitation signal is a sinusoidal current or sinusoidal voltage signal with a preset frequency. <10Hz.

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