A method for monitoring the cycle capacity of lithium-ion power batteries based on pressure characteristics

Abstract

The present invention relates to the field of energy storage, and provides a method for monitoring the cycle capacity of a lithium-ion power battery based on pressure characteristics. The specific method includes, for a target battery, collecting the pressure signal generated by volume expansion and contraction during the battery cycle; performing noise reduction processing on the obtained pressure signal, and dividing each cycle according to the pressure signal after noise reduction; calculating the reversible expansion force and reversible contraction force generated by each cycle of the battery; and calculating the charge and discharge capacity of the battery based on the reversible expansion force, reversible contraction force and mechanical parameter input. The method proposed in the present invention takes into account the actual assembly conditions and the mechanical properties of the battery, avoids the influence of irreversible expansion on pressure measurement, and provides a practical and feasible method for capacity monitoring of the battery during actual use.

Description

Technical Field

[0001] The present invention relates to the field of energy storage and provides a method for monitoring the cycle capacity of a lithium-ion power battery based on pressure characteristics. Technical Background

[0002] Lithium-ion batteries are widely used in electronic products, electric vehicles, and large-scale energy storage systems due to their high energy density, long cycle life, and low self-discharge rate. However, range anxiety has always been an urgent problem that needs to be solved for the further development of lithium batteries. Lithium batteries experience capacity decay during the cycle process due to factors such as the decay of electrode materials, the decomposition of electrolytes, and the formation of solid electrolyte interface (SEI) films. Capacity decay not only leads to a decrease in the energy storage performance of the battery, but also increases the risk of thermal runaway and fire accidents. Therefore, it is of great significance to accurately monitor the battery capacity.

[0003] Currently, the common capacity detection methods are cyclic voltammetry and constant current charge and discharge test. The former analyzes electrochemical properties through voltage-current curves, which is highly sensitive but complicated to operate; the latter is simple to operate but time-consuming. Impedance spectroscopy (EIS) quickly and non-destructively evaluates the internal characteristics of the battery by measuring changes in AC impedance, but the data analysis is complex. Accelerated aging tests predict battery life under extreme conditions, and the results may differ from actual use. In addition, capacity prediction methods based on models and neural networks are also commonly used to detect battery capacity, but this method relies on the accuracy of the model and training data and cannot accurately reflect the true capacity of the battery.

[0004] During the cyclic use, the battery capacity gradually decays, and the thickness change of the electrode material also decreases. The insertion and extraction of lithium ions causes the volume of the electrode material to expand and contract, but as the number of cycles increases, the electrode material gradually suffers from mechanical and electrochemical stresses, resulting in irreversible changes such as cracking and pulverization, which reduces its carrying capacity for lithium ions. Therefore, the battery capacity decay is accompanied by a decrease in the electrode thickness change, reflecting the dynamic changes in the structure and composition of the electrode material. However, detecting the thickness change of the battery requires extremely high accuracy, and the high mechanical strength of the battery casing makes measurement difficult. Therefore, the battery expansion volume can be limited by adding external fixed constraints, and the thickness change can be converted into pressure for detection.

[0005] At present, relevant studies have proved the relationship between battery capacity and reversible expansion (The Electrochemical Society, 2021, 168 (10): 100520.). At the same time, relevant invention patents have proposed methods for estimating the SOC of batteries and predicting the life of batteries based on pressure by using pressure to react to the charge state of batteries (CN202210605483.2, CN202210602373.0, CN202410108486.4 and CN202110004797.2). It should be noted that the capacity detection method proposed in the present invention is different from the above-mentioned existing methods. Its creativity lies in that the method proposed in the present invention avoids the influence of irreversible expansion of batteries on pressure measurement from a theoretical perspective, and takes into account the mechanical performance parameters of the fixture and the battery in the calculation method, providing a practical and feasible method for capacity monitoring of batteries during actual use. Summary of the invention

[0006] The present invention proposes a method for monitoring the cycle capacity of lithium-ion power batteries based on pressure characteristics. The method aims to accurately evaluate the battery capacity by real-time monitoring the volume change of the battery during the charging and discharging process, thereby improving the performance of the battery management system and the battery life. The specific steps are as follows:

[0007] Step 1: For a target battery, collect the pressure signal generated by volume expansion and contraction during the battery cycle;

[0008] Step 2, performing noise reduction processing on the pressure signal obtained in step 1, and dividing each cycle according to the noise-reduced pressure signal;

[0009] Step 3, calculating the reversible expansion force and reversible contraction force generated by each cycle of the battery;

[0010] Step 4: Calculate the charge and discharge capacity of the battery based on the change characteristics of the reversible expansion force and reversible contraction force and the mechanical parameter input.

[0011] Preferably, the target battery is a commercial button cell, a square cell and a soft pack cell, or a homemade button cell, a square cell or a soft pack cell.

[0012] Preferably, the method is applied under the condition that the battery expansion is completely limited, that is, a rigid fixture is placed in the vertical direction of the electrode expansion, the fixture does not move due to the volume change of the battery, and a compression force perpendicular to the battery surface is applied to the battery in actual assembly;

[0013] Optionally, the noise reduction processing method includes, but is not limited to, moving average filtering method, wavelet transform denoising method, machine learning and other noise reduction methods. The signal after noise reduction should significantly reduce random noise and burst noise, making the signal smoother and more continuous. This step is a necessary step for subsequent signal processing.

[0014] Preferably, in step two, each cycle is divided according to the pressure signal after noise reduction. The division method is to calculate the peak value and valley value of the pressure signal, and record one cycle from one peak value to the next peak value.

[0015] p i,peak ={x|x(t)>x(t - t0)+α and x|x(t)>x(t + t0)+α} (1)

[0016] p i,valley ={x|x(t)<x(t - t0)+β and x|x(t)<x(t + t0)+β} (2)

[0017] Where x is the pressure signal value; t is the time; t0 is the measurement interval time; α is the peak factor; β is the valley factor; p i,peak is the peak value of one cycle, reaching this value indicates the end of charging for this cycle; p i,valley is the valley value of one cycle, reaching this value indicates the end of discharging for this cycle.

[0018] Preferably, the method for calculating the reversible expansion force and reversible contraction force generated by the battery in each cycle is as follows:

[0019] △p i,c =p i,peak -p i,valley (3)

[0020] △p i,d =p i,valley -p i,peak (4)

[0021] Where △p i,c is the reversible expansion force when the battery is charged in the i-th cycle; △p i,d is the reversible contraction force when the battery is discharged in the i-th cycle;

[0022] Preferably, the specific method for calculating the capacity is as follows:

[0023] Q i,c =a·s i,c (5)

[0024] Q i,d =a·s i,d (6)

[0025] Where Q i,cis the charge capacity for one cycle, Q i,d is the discharge capacity of one cycle; a is a constant, which can be obtained according to the actual measurement of the target battery; s i,c The displacement of the battery during this cycle charging, s i,d This is the displacement of the battery when it is compressed during charging in this cycle. i,c Calculated according to the following relationship:

[0026] k s (s i,c +s0)+k c s i,c =k a1 (s f -s i,c )+k a2 (s f -s i,c ) 3 (7)

[0027] where k s is the equivalent stiffness of the fixture; k c is the equivalent stiffness of the battery casing; k a1 is the equivalent stiffness of the battery core linear term; k a2 is the equivalent stiffness of the nonlinear term of the battery core; the above parameters can be obtained through mechanical properties testing; s f is the displacement of the winding core under the charge and discharge conditions; s0 is the compressed thickness of the battery and the fixture under the pressure. f and s0 are calculated according to the following relations:

[0028]

[0029]

[0030] in, is the expansion displacement of the battery under unrestricted conditions, which can be obtained through actual measurement; 0c is the compressed thickness of the battery under the current pressure, which can be calculated by the following relationship:

[0031]

[0032] where △p i Substitute the value △p calculated in the above steps into i,c .

[0033] s i,d With s i,c The calculation method is the same, △p i Substitute the value △p calculated in the above steps into i,d . BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A method and steps for monitoring the cycle capacity of a lithium-ion power battery based on pressure characteristics;

[0035] Figure 2 Schematic diagram of the relative positions of the battery and the sensor in the fixed constraint;

[0036] Figure 3 Schematic diagram of peak and valley values ​​in the pressure signal after noise reduction;

[0037] Figure 4 Schematic diagram of the relationship between battery capacity and reversible pressure change range. DETAILED DESCRIPTION

[0038] In order to make the technical problems, technical solutions and beneficial effects to be solved by the embodiments of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0039] Some of the following methods and embodiments of the present invention are based on the following understanding:

[0040] The volume change of lithium-ion batteries is closely related to the electrochemical reactions inside them. During the charging process, lithium ions are embedded from the positive electrode into the negative electrode, causing the volume of the negative electrode material to expand, resulting in an increase in the thickness of the battery; during the discharging process, lithium ions are released from the negative electrode and returned to the positive electrode, causing the volume of the negative electrode material to shrink, resulting in a decrease in the thickness of the battery. In addition, the temperature rise caused by the ohmic heat and polarization heat generated by the battery charging and discharging will cause thermal expansion of the internal materials of the battery, resulting in an increase in the thickness of the battery. When lithium is deposited inside the battery, the SEI film grows, or gas is produced inside, the battery will swell abnormally.

[0041] During the cyclic use, the battery capacity gradually decays, and the thickness change of the electrode material also decreases. The insertion and extraction of lithium ions causes the volume of the electrode material to expand and shrink, but as the number of cycles increases, the electrode material gradually suffers from mechanical and electrochemical stresses, resulting in irreversible changes such as cracks and pulverization, which reduces its carrying capacity for lithium ions. At present, relevant studies have demonstrated the relationship between battery capacity and reversible expansion. The battery capacity decay is accompanied by a decrease in the electrode thickness change, reflecting the dynamic changes in the structure and composition of the electrode material.

[0042] However, detecting the thickness change of the battery requires extremely high accuracy, and the high mechanical strength of the battery casing makes measurement difficult. Therefore, by adding external fixed constraints, the battery expansion volume can be limited, and the thickness change can be converted into pressure for detection. According to Hooke's law, the relationship between battery volume change and pressure can be expressed as a linear relationship. Therefore, by measuring the change in pressure, the electrochemical state inside the battery can be obtained.

[0043] Based on the above understanding, the present invention provides the following embodiments:

[0044] Example 1

[0045] During the cyclic charge and discharge process of lithium-ion power batteries, the pressure signal is collected and noise-reduced, and each cycle node is divided.

[0046] Select a high-precision thin-film pressure sensor that has good long-term stability and linearity. Figure 2 , the thin film sensor is installed between the battery and the rigid fixture. During assembly, the initial pressure applied by the rigid material to the battery is 115N. This embodiment records the battery pressure changes over 85 hours. During this period, the battery is charged and discharged at a rate of 0.5C, and the sensor records the pressure signal generated by the expansion and contraction of the battery volume. Further, the raw pressure signal is obtained from the sensor, and the signal is denoised using an exponential moving average filter. The denoised signal is shown in Figure 3 . Calculate the peak and valley values ​​of the pressure signal. Define a window size of 5s and find the maximum value in the window. Figure 3 The peak and valley values ​​of each cycle are shown, with the peak value being the beginning of the cycle.

[0047] Example 2

[0048] Mechanical parameters of this embodiment are selected. The mechanical parameters of the battery obtained according to some experimental tests and references are listed in Table 1.

[0049]

[0050]

[0051] Example 3

[0052] The charge capacity of the battery in one cycle was calculated using the reversible expansion force of the battery and the mechanical parameters in Table 1.

[0053] First, the reversible expansion force of the battery can be calculated by formula (3). When the charging start pressure is 102N and the charging end pressure is 51N, the reversible expansion force of the battery △p i,c is 51N; according to formula (10), s 0c 1.44×10 -5 According to formula (8), s0 can be calculated to be 5.59×10 -5 , calculated according to formula (9) f is 0.0016; according to formula (7), s i,c is 0.0018, and the battery capacity Q is calculated according to formula (5):i,c The result of the experiment is 32.97Ah. To verify the accuracy of the model output, Figure 4 The measured experimental values ​​and model predictions at different pressures are provided.

Claims

1. A method for monitoring the cycle capacity of a lithium-ion power battery based on pressure characteristics, characterized in that: The method specifically comprises the following steps: Step 1: Collect the pressure signal generated by the volume expansion and contraction of the battery during the cycle, and divide each cycle after the signal noise reduction process. The division method is to calculate the peak and valley values ​​of the pressure signal, and record the period from one peak to the next peak as one cycle; Step 2: Calculate the reversible expansion force and reversible contraction force generated in each cycle of the battery as follows: △p i,c =p i,peak -p i,valley △p i,d =p i,valley -p i,peak where p i,peak is the peak value of the i-th cycle, reaching this value indicates the end of the cycle charging; p i,valley is the valley value of the i-th cycle, reaching this value indicates the end of the discharge cycle; where △p i,c is the reversible expansion force of the battery during charging in the i-th cycle; △p i,d is the reversible contraction force of the battery during discharge in the i-th cycle; Step 3: According to the changing characteristics of the reversible expansion force and the reversible contraction force, combined with the mechanical parameters, the charge and discharge capacity of the battery is calculated. The specific calculation method is as follows: Q i,c =a·s i,c ;Q i,d =a·s i,d ; Where Q i,c is the charge capacity for one cycle, Q i,d is the discharge capacity of one cycle; a is a constant, which can be obtained according to the actual measurement of the battery; s i,c The displacement of the battery during this cycle charging, s i,d The displacement of the battery during this cycle charging, s i,c Calculated according to the following relationship: k s (s i,c +s0)+k c s i,c =k a1 (s f -s i,c )+k a2 (s f -s i,c ) 3 ; where k s is the equivalent stiffness of the fixture; k c is the equivalent stiffness of the battery casing; k a1 is the equivalent stiffness of the battery core linear term; k a2 is the equivalent stiffness of the nonlinear term of the battery core; s f is the displacement of the winding core under the charge and discharge conditions; s0 is the compressed thickness of the battery and the fixture under the pressure, s f and s0 are calculated according to the following relations: in, is the expansion displacement of the battery under unrestricted conditions, which can be obtained through actual measurement; 0c is the compressed thickness of the battery under the current pressure, which can be calculated by the following relationship: Among them, △p i Substitute △p i,c , calculate s i,c ; △p i Substitute △p i,d , calculate s i,d .

2. The method for monitoring the cycle capacity of a lithium-ion power battery based on pressure characteristics according to claim 1, characterized in that: The battery in step one is a rectangular parallelepiped structure, specifically including a square battery and a soft-pack battery.

3. According to the method for monitoring the cycle capacity of a lithium-ion power battery based on pressure characteristics as described in claim 1, it is characterized in that: This method is applied under the condition that the battery expansion is completely restricted, that is, a rigid clamp is placed in the direction perpendicular to the electrode expansion, and the clamp does not move due to the volume change of the battery. In actual assembly, a compression force perpendicular to the battery surface is applied to the battery.

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