Lithium-ion battery module micro-overcharge detection method based on pressure characteristics

Abstract

The present invention belongs to the field of energy storage, and proposes a method for detecting micro-overcharge of lithium-ion battery modules based on pressure characteristics. The core of the method is to monitor the pressure changes of each battery cell in the operation process of the battery module in real time through a pressure sensor, so as to identify the micro-overcharge phenomenon. The specific steps include: pressure measurement, that is, during the charging and discharging process of the battery module, using a pressure sensor to measure the pressure signal of each battery cell under rigid constraints; signal processing, noise reduction processing of the collected pressure signal to ensure the accuracy and reliability of the data; feature extraction: calculate and extract the pressure signal characteristics of each battery cell to determine whether each battery cell has a micro-overcharge. The present invention effectively solves the problem that it is difficult to detect micro-overcharge of individual batteries in the battery module, and provides an accurate and real-time monitoring method, which helps to improve the safety and service life of the battery module.

Description

Technical Field

[0001] The invention relates to the field of energy storage and provides a method for detecting slight overcharge of a lithium-ion battery module based on pressure characteristics. Technical Background

[0002] Lithium-ion batteries have been 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 and safety issues remain key challenges that need to be addressed in the further development of lithium batteries. Due to the inconsistency of battery cells, individual batteries in the battery module are often slightly overcharged. Micro-overcharging of the battery will accelerate the attenuation of the electrode material, reduce thermal stability, and induce lithium precipitation. In severe cases, lithium dendrites may pierce the solid electrolyte interface (SEI) membrane, causing an internal short circuit in the battery. Therefore, micro-overcharging not only leads to capacity decay and performance degradation of lithium batteries, but also significantly increases the risk of thermal runaway and fire accidents.

[0003] At present, the micro-overcharge monitoring methods mainly include voltage monitoring, current monitoring, temperature monitoring, electrochemical impedance spectroscopy (EIS) analysis, online monitoring systems and gas sensors. These methods have their own advantages. For example, voltage and current monitoring are direct and simple, temperature monitoring and gas sensors can indirectly reflect the battery status, and EIS analysis provides detailed electrochemical characteristic data. However, these methods also have shortcomings. Voltage and current monitoring are insensitive to slight overcharges, temperature monitoring and gas sensors have a delayed response, EIS analysis is complex and requires special equipment, and online monitoring systems are expensive and require multi-parameter coordination. Therefore, there is an urgent need for a direct and reliable method to detect micro-overcharge of batteries.

[0004] The pressure signal is an important signal reflecting the battery status. Previous researchers have proposed methods to estimate the battery SOC and predict the battery life through battery pressure (CN202210605483.2, CN202210602373.0 and CN202110004797.2). However, there is still no effective method to detect battery micro-overcharge based on the characteristics of the pressure signal.

[0005] The present invention proposes a lithium-ion battery module micro-overcharge detection method based on pressure characteristics by timely responding to the lithium deposition and charging capacity inside the battery by the pressure signal under fixed constraints, so as to solve the problem that the micro-overcharge of single cells in the battery module is difficult to detect. Summary of the invention

[0006] The present invention discloses a lithium-ion battery module micro-overcharge detection method based on pressure characteristics, which identifies and prevents the micro-overcharge phenomenon of the battery by analyzing the pressure signal of the battery during the charging and discharging process, so as to ensure the safety of the battery and extend its service life.

[0007] Preferably, the method should be implemented using a battery module pressure measuring device, which has the following characteristics: the partition in the middle of each battery cell in the battery module, the support plates at both ends of the battery module, and the connection between the partition, the support plate and the bottom of the battery module are all immovable fixed connectors. The specific steps of the method are as follows:

[0008] Step 1: During the charging and discharging process of the battery module, the pressure signal generated by each battery cell is collected and noise reduction is performed;

[0009] Step 2, calculating the first-order derivative of the pressure signal in time;

[0010] Step 3: Calculate the charging pressure range as follows:

[0011] △F i =F i.e -F i,0 (1)

[0012] Among them, △F i is the charging pressure range of the i-th cycle; F i.e F is the pressure at the end of the i-th cycle charge; i,0 is the pressure at the beginning of charging of the i-th cycle.

[0013] Step 4, identify the characteristics of the battery's slightly overcharge pressure: Characteristic 1, the battery cell has an irreversible expansion phenomenon, which is manifested as the pressure value at the beginning of each cycle charging increases with the number of cycles. At this time, it is considered that there is a possibility of lithium deposition, solid electrolyte interface film and a small amount of gas inside the battery; Characteristic 2, the pressure difference signal exceeds a certain value, at this time it is considered that the battery begins to deposit lithium, and this value is called the lithium deposition threshold; Characteristic 3, the charging pressure interval exceeds a certain value, at this time it is considered that the battery is charged beyond its standard capacity, and this value is called the capacity threshold;

[0014] Preferably, the lithium plating threshold and the capacity threshold should be determined according to the specific battery model, and the determination steps are as follows: select a standard battery of the same model as the test battery, and apply an initial pressure equivalent to that in the actual assembly; after the battery is force balanced, perform a charge and discharge cycle test on the battery at a charge and discharge rate of 0.5C, and measure the pressure generated during the process; calculate the pressure difference signal, and record the maximum value of the pressure difference signal as the lithium plating threshold; calculate the charging pressure interval, and record the maximum value of the charging pressure interval as the interval threshold.

[0015] Through the above method, it is possible to effectively detect whether there is a slight overcharge phenomenon in individual batteries of the lithium-ion battery module during the charging and discharging process, and then take necessary measures to prevent battery damage and safety accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1Schematic diagram of the steps of the lithium-ion battery module micro-overcharge detection method based on pressure characteristics;

[0017] Figure 2 Schematic diagram of the battery module pressure measurement device;

[0018] Figure 3 Schematic diagram of pressure signal charging interval calculation and capacity threshold;

[0019] Figure 4 Schematic diagram of the calculation of the first-order derivative of the pressure signal and the lithium precipitation threshold;

[0020] Figure 5 Flow chart of the method for identifying micro-overcharge pressure characteristics;

[0021] Figure 6 CT scan of the battery cell after slight overcharging. DETAILED DESCRIPTION

[0022] 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.

[0023] Some of the following methods and embodiments of the present invention are based on the following understanding:

[0024] 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.

[0025] In the case of a slight overcharge of a lithium-ion battery, the battery capacity and electrode thickness will change significantly. In the early stage, slight overcharging can lead to a slight increase in battery capacity because more lithium ions are embedded in the positive electrode material. However, long-term or frequent slight overcharging can cause irreversible damage such as electrolyte decomposition and structural destruction of the positive electrode material. In addition, in the case of overcharging, excess lithium ions cannot be completely embedded in the negative electrode material, but are precipitated on the surface of the negative electrode, while producing trace gases, increasing the thickness of the negative electrode and the overall volume of the battery cell, causing the battery cell to show irreversible expansion.

[0026] 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.

[0027] Based on the above understanding, the present invention provides the following embodiments:

[0028] Example 1

[0029] The basic steps of this embodiment are as follows: Figure 1 shown.

[0030] During the charging and discharging process of the battery module, the pressure signal generated by each battery cell is collected. This embodiment should use a battery module pressure measurement device (such as Figure 2 ), the partitions in the middle of each battery cell in the battery module, the support plates at both ends of the battery module, and the connection between the partitions, support plates and the bottom of the battery module are all immovable fixed connectors. The pressure sensor is attached to the surface of the battery with a thickness of less than 2mm to avoid local pressure on the battery in the rigid constraint. The obtained pressure signal is denoised using the wavelet packet denoising method to ensure the authenticity and reliability of the data.

[0031] The charging pressure range is calculated according to formula (1). The calculation result is as follows: Figure 3 Calculate the first-order derivative of the pressure signal in time, and the calculation result is as follows Figure 4 Identify whether the battery pressure characteristics are slightly overcharged. Figure 5 The specific steps of the identification method are shown: determine whether the battery cell has irreversible expansion. If the pressure value at the beginning of each cycle increases with the number of cycles, it is considered that there is a possibility of lithium deposition, solid electrolyte interface film and a small amount of gas inside the battery; determine whether the first-order derivative of the pressure exceeds the threshold. If it exceeds the lithium deposition threshold, it is considered that the battery has begun to deposit lithium. Determine whether the charging pressure range exceeds the threshold. If it exceeds the capacity threshold, it is considered that the battery is charged beyond its standard capacity. Finally, it is considered that the battery is slightly overcharged. Figure 6 This is an X-ray CT scan of a slightly overcharged battery. The battery has irreversible expansion after being slightly overcharged.

[0032] Example 2

[0033] The lithium precipitation threshold and capacity threshold should be determined according to the specific battery model. The determination steps are as follows: select a standard battery of the same model as the test battery and apply the same initial pressure as in the actual assembly; after the battery is balanced by force, perform a charge and discharge cycle test on the battery at a charge and discharge rate of 0.5C and measure the pressure generated during the process; calculate the pressure difference signal and record the maximum value of the pressure difference signal as the lithium precipitation threshold (such as Figure 4 ); calculate the charging pressure interval, and record the maximum value of the charging pressure interval as the capacity threshold (such as Figure 3 ).

Claims

1. A lithium-ion battery module micro-overcharge detection method based on pressure characteristics, characterized in that: The method specifically comprises the following steps: S1: During the charging and discharging process of the battery module, the pressure signal generated by each battery cell is collected and noise reduction is performed; S2: Calculate the pressure difference signal, that is, the first-order derivative of the pressure signal in time; S3: Calculate the charging pressure range, the calculation method is: △F i =F i.e -F i,0 ; Among them, △F i is the charging pressure range of the i-th cycle; F i.e F is the pressure at the end of the i-th cycle charge; i,0 is the pressure at the beginning of the i-th cycle charging; S4: Identify the battery slightly overcharge state based on the pressure characteristics. The specific identification method includes the following steps: S41: The battery cell has an irreversible expansion phenomenon, which is manifested as the pressure value at the beginning of each cycle charging increases with the number of cycles. At this time, it is believed that there is a possibility of lithium deposition, solid electrolyte interface film and a small amount of gas generation inside the battery; S42: lithium deposition determination, by comparing the maximum value of the pressure difference of the pressure signal of the battery to be tested with a pre-calibrated lithium deposition threshold, when the maximum value of the pressure difference exceeds the lithium deposition threshold, determining that lithium deposition occurs in the battery; S43: overcharge determination, by comparing the maximum value of the charging pressure interval of the battery to be tested with a pre-calibrated capacity threshold, when the maximum value of the charging pressure interval exceeds the capacity threshold, determining that the battery is in an overcharge state; Among them, the calibration method of the lithium plating threshold and the capacity threshold includes: selecting a standard battery sample of the same model as the battery to be tested; applying the same initial pressure constraint conditions as the actual assembly conditions to the standard battery sample; after the battery system reaches a mechanical equilibrium state, collecting pressure parameters under 0.5C constant current charge and discharge cycle conditions; based on the test data of the standard battery sample, determining that the lithium plating threshold and the capacity threshold are the maximum values ​​of the pressure difference signal and the charging pressure range of the standard battery, respectively.

2. According to the pressure characteristic-based lithium-ion battery module micro-overcharge detection method described in claim 1, it is characterized in that: The pressure signals generated by each battery cell in step S1 should be collected using a battery module pressure measuring device. Its characteristics are as follows: the partition in the middle of each battery cell in the battery module, the support plates at both ends of the battery module, and the connection between the partition, the support plate and the bottom of the battery module are all immovable fixed connecting parts; before measurement, the initial pressure of each single battery should be adjusted not to exceed the commonly used pressure range.

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