Method for analyzing internal pressure of square battery based on surface stress change

Abstract

The invention discloses a method for analyzing internal pressure of a square battery based on surface stress change, which measures the change of the internal pressure and the surface stress of the battery along with time in real time through a thin film array pressure sensor and a pressure sensor with a screwed joint. And on the basis of fitting analysis of curves of the internal pressure and the surface stress changing along with time, a function relational expression between the internal pressure and the surface stress of the square battery is constructed. Therefore, when the internal pressure of other square batteries to be explained is analyzed, the internal pressure change of the battery can be accurately predicted only by acquiring the surface stress of the square battery. According to the method, the internal pressure of the square battery can be obtained without destructive treatment on the battery shell, and compared with a traditional method for measuring the internal pressure through trepanning, the operation difficulty is greatly reduced, and meanwhile, the safety of the measurement process and the reliability of data are effectively guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery internal pressure monitoring, and in particular to a method for analyzing the internal pressure of a square battery based on surface stress changes. Background Art

[0002] During operation, complex electrochemical reactions within lithium-ion batteries can trigger side reactions and gas generation, leading to a gradual increase in internal battery pressure and ultimately causing the battery casing to expand. This is particularly true under extreme operating conditions, such as overcharge, over-discharge, high temperatures, or mechanical abuse. These side reactions, such as electrolyte decomposition, rupture of the negative electrode SEI film, and oxidation of the positive electrode material, can intensify. These reactions can lead to the generation of large amounts of gas, causing a sharp increase in internal battery pressure.

[0003] Fluctuations in battery internal pressure are closely linked to safety performance. Excessive internal pressure can cause cracks in the battery casing, seal failure, or even trigger the safety valve to open, releasing flammable gases and leading to more severe thermal runaway. Thermal runaway is an irreversible chain reaction, accompanied by a dramatic temperature rise and gas release, which can ultimately cause the battery to combust or explode, posing a significant threat to battery system safety.

[0004] However, directly measuring the internal pressure of a battery is technically difficult. Existing techniques typically measure internal pressure changes by drilling holes in the battery casing and placing sensors there. However, this method can easily damage the battery's internal structure, affecting its sealing, leading to electrolyte leakage or air ingress, which in turn affects the measurement results. Summary of the Invention

[0005] The present invention discloses a method for analyzing the internal pressure of a square battery based on surface stress changes to overcome the above technical problems.

[0006] In order to achieve the above object, the technical solution of the present invention is:

[0007] A method for analyzing the internal pressure of a square battery based on surface stress changes includes the following steps:

[0008] S1: A thin film array pressure sensor is placed on the surface of the square battery and a test preload is applied;

[0009] S2: A pressure sensor with a threaded connector is fixedly connected to the square battery;

[0010] S3: heating the square battery to obtain the surface stress of the square battery through the thin film array pressure sensor and the internal pressure of the square battery through the pressure sensor with a threaded connector before the battery safety valve is opened; thereby obtaining a curve showing the surface stress of the square battery and the internal pressure of the square battery varying with time;

[0011] S4; using a nonlinear exponential function, fitting the curve of the surface stress of the square battery changing with time and the curve of the internal pressure of the square battery changing with time, respectively, to determine the expression relationship between the surface stress of the square battery and time after fitting and the expression relationship between the internal pressure of the square battery and time after fitting;

[0012] S5: Obtaining a functional relationship between the internal pressure and the surface stress of the square battery according to the fitted expression of the surface stress and time of the square battery and the fitted expression of the internal pressure and time of the square battery;

[0013] S6: A thin film array pressure sensor is set on the square battery to be analyzed, and a preload force equal to the test preload force is applied; the square battery to be analyzed is heated to obtain the surface stress of the square battery to be analyzed, so as to obtain the internal pressure of the square battery to be interpreted based on the functional relationship between the internal pressure and the surface stress of the square battery, so as to realize the analysis of the internal pressure of the square battery.

[0014] Furthermore, in S5, the functional relationship between the internal pressure and the surface stress of the square battery is expressed as follows:

[0015]

[0016] Where: y2 represents the internal pressure of the square battery; C2 represents the baseline level of the initial internal pressure; A2 represents the initial response intensity of the internal pressure; y1 represents the surface stress of the square battery; A1 represents the initial response intensity of the surface stress; C1 represents the baseline level of the initial surface stress; B1 represents the rate of increase of the surface stress over time; B2 represents the rate of increase of the internal pressure over time.

[0017] Furthermore, the surface of the square battery on which the thin film array pressure sensor is arranged is the surface with the largest area on the square battery.

[0018] Furthermore, a method for fixing a pressure sensor with a threaded connector to the square battery is as follows:

[0019] S201: Setting a threaded hole on the surface of the square battery;

[0020] S202: Using the external thread provided on the pressure sensor with the threaded joint to connect with the threaded hole and tighten;

[0021] S203: Seal the position where the threaded hole is connected and fastened.

[0022] Furthermore, in S4, the fitted expression of the surface stress and time of the square battery is expressed as follows:

[0023] y1=A1×exp(t / B1)+C1

[0024] Where: y1 represents the surface stress of the square battery; A1 represents the initial response intensity of the surface stress; exp(·) represents the exponential function; B1 represents the rate at which the surface stress increases with time; C1 represents the baseline level of the initial surface stress; t represents time;

[0025] The fitted relationship between the internal pressure and time of the square battery is expressed as follows:

[0026] y2=A2×exp(t / B2)+C2

[0027] Where: y2 represents the internal pressure of the square battery; A2 represents the initial response intensity of the internal pressure; B2 represents the rate at which the internal pressure increases over time; C2 represents the baseline level of the initial internal pressure.

[0028] Beneficial effects: The present invention provides a method for analyzing the internal pressure of a square battery based on surface stress changes. The method places a thin film array pressure sensor on the large surface of the square battery and fixes it to the square battery through a pressure sensor with a threaded joint. The thin film array pressure sensor and the pressure sensor with a threaded joint are used to measure the changes in the internal pressure and surface stress of the battery over time in real time. Since the increase in internal pressure directly affects the change in surface stress, the change in internal pressure can be reflected by measuring the change in battery surface stress. Based on the fitting analysis of the curves of internal pressure and surface stress changing with time, a functional relationship between the internal pressure and surface stress of the square battery is constructed. Therefore, when analyzing the internal pressure of other square batteries to be interpreted, the internal pressure change of the battery can be accurately predicted by only obtaining the surface stress of the square battery. The present invention can obtain the internal pressure of the square battery without destructive treatment of the battery shell. Compared with the traditional method of measuring internal pressure by opening a hole, the operation difficulty is greatly reduced, while effectively ensuring the safety of the measurement process and the reliability of the data. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0030] Figure 1This is a flow chart of the method for analyzing internal pressure changes of a square battery based on surface stress changes in the present invention;

[0031] Figure 2 Schematic diagram of the point distribution of the thin film pressure sensor in an embodiment of the present invention;

[0032] Figure 3 Schematic diagram of experimental battery arrangement scheme in an embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram of installing a pressure sensor with a threaded connector in an embodiment of the present invention;

[0034] Figure 5 This is a schematic diagram of a function expression obtained by fitting the surface stress and internal pressure curves in an embodiment of the present invention;

[0035] Figure 6 This is a schematic diagram of the functional relationship between internal pressure and surface stress derived from the surface stress and internal pressure functional expressions in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0037] This embodiment introduces a method for analyzing the internal pressure of a square battery based on surface stress changes, including the following steps: Figure 1 As shown:

[0038] S1: A thin film array pressure sensor is placed on the surface of the square battery and a test preload is applied;

[0039] Preferably, the surface of the square battery on which the thin film array pressure sensor is provided is the surface with the largest area on the square battery, which can monitor the surface stress changes before the safety valve is opened in real time and facilitate more accurate acquisition of stress data and distribution on the battery surface.

[0040] Specifically, this embodiment uses an ultra-thin, flexible and bendable thin film array pressure sensor, which is composed of a double layer of flexible material, wherein the upper layer is a functional film with pressure-sensitive properties for sensing stress changes, and the lower layer is a conductive electrode film for signal transmission and acquisition, which can accurately cover the entire large surface of the battery (that is, the surface with the largest area of ​​the square battery). At the same time, a steel clamp is used to apply a certain amount of test pre-tightening force to make the square battery in close contact with the thin film sensor, ensuring that the thin film array pressure sensor can record the contact stress distribution of the battery surface in real time; the steel clamp adopts a conventional clamp structure in this field, which is two equal-sized rectangular blocks equipped with 4 bolts of equal length to achieve fixation between the thin film array pressure sensor and the large surface of the square battery.

[0041] The thin film array pressure sensor can also adjust the data acquisition frequency to accurately capture the rapid changes in stress during battery expansion under different working conditions.

[0042] In this embodiment, based on the size of the large surface of the square battery, the length and height of the battery are measured, and a thin film array pressure sensor of the same size is customized. The number of required measurement points on the large surface of the square battery is set according to actual needs. In this embodiment of the present invention, the thin film array pressure sensor is evenly divided into 5×6 areas, forming a total of 30 equal-sized measurement points. Each point can independently and synchronously detect the stress changes in the corresponding area, such as Figure 2 As shown. Put the lithium iron phosphate battery, heating plate, pressure film sensor, thermal insulation cotton, steel fixture and other equipment into Figure 3 Assemble in the order shown and apply pre-tightening force to ensure close contact between the thin film pressure sensor and the battery surface. The battery 1 is set in the middle layer. On one side of the battery, the thin film pressure sensor 4, thermal insulation cotton 2, and steel clamp 3 are set in order from the inside to the outside. On the other side of the battery, the heating plate 5, thermal insulation cotton 2, and steel clamp 3 are set in order from the inside to the outside. Figure 3 shown.

[0043] S2: A pressure sensor with a threaded connector is fixedly connected to the square battery;

[0044] Preferably, the method for fixing the pressure sensor with a threaded joint to the square battery is as follows:

[0045] S201: Setting a threaded hole on the surface of the square battery;

[0046] S202: Using the external thread provided on the pressure sensor with the threaded joint to connect with the threaded hole and tighten;

[0047] S203: Sealing is performed at the position where the threaded hole is docked and fastened.

[0048] Specifically, the pressure sensor with threaded holes in this embodiment is an existing product in the field. Real-time acquisition of changes in the internal pressure of the battery during the expansion process; Figure 4 As shown, the pressure sensor with a threaded connector is installed on a square battery with a threaded hole. After the connection is completed, it is sealed with high-temperature resistant thread fastening glue and left to stand for 24 hours.

[0049] Specifically, since it is difficult to open a hole in the battery and it is easy to cause damage to the inside of the battery, in this embodiment, the pressure sensor is connected to the square battery housing by converting a G1 / 8 external thread to a 1 / 8-inch ferrule joint. Among them, the square battery housing is preset with an internal thread interface that meets the G1 / 8 thread standard, and the pressure sensor is docked and tightened through the external thread. It is then connected to the pressure sensor through a 1 / 8-inch stainless steel ferrule, and a double ferrule structure (front ferrule and rear ferrule) is used to achieve reliable mechanical locking and sealing to ensure that the gas channel does not leak under high-pressure conditions. In order to enhance the sealing performance of the connection part, the joint is supplemented with polytetrafluoroethylene (PTFE) sealing tape or liquid sealant to prevent leakage of gas or electrolyte inside the battery during thermal runaway. At the same time, this connection method has good versatility and vibration resistance, and is suitable for dynamic pressure monitoring under thermal runaway conditions.

[0050] Specifically, in this embodiment, the threaded hole for fastening the pressure sensor with the threaded hole is arranged next to the safety valve. When the battery is opened and assembled, the opening and assembly are generally performed in an inert gas environment such as argon to prevent air from entering the battery and affecting the battery structure. After the arrangement is completed, it is sealed with a high-temperature resistant thread fastening glue and left to stand for 24 hours.

[0051] Specifically, the threaded hole is generally opened on the upper substrate of the battery, and does not share the same hole with the safety valve. It is generally located adjacent to the safety valve. The diameter of the threaded hole should not be too large, generally 2 to 3 mm, to ensure that the internal pressure change can be detected when the fixing fixture applies pre-tightening force. If the hole is too large, it is easy for air to enter the battery, causing changes in the internal environment. The depth only needs to slightly pierce the battery substrate to prevent the thread from piercing the internal winding core of the battery when tightening the connection, causing a micro short circuit.

[0052] S3: heating the square battery to obtain the surface stress of the square battery through the thin film array pressure sensor and the internal pressure of the square battery through the pressure sensor with a threaded connector before the battery safety valve is opened; thereby obtaining a curve showing the surface stress of the square battery and the internal pressure of the square battery varying with time;

[0053] Specifically, since the thermal runaway warning for the battery mainly focuses on the early changes in the internal pressure of the battery, the internal pressure of the battery changes very little after the safety valve is opened. Therefore, this embodiment is based on the situation before the battery safety valve is opened.

[0054] In this embodiment, the square battery is heated by a heating plate with constant power and the same size as the battery surface to achieve single-sided heating, which is placed away from the side where the pressure film sensor is attached. Figure 3 shown.

[0055] S4; using a nonlinear exponential function, fitting the curve of the surface stress of the square battery changing with time and the curve of the internal pressure of the square battery changing with time, respectively, to determine the expression relationship between the surface stress of the square battery and time after fitting and the expression relationship between the internal pressure of the square battery and time after fitting;

[0056] Preferably, the fitted relationship between the surface stress and time of the square battery is expressed as follows:

[0057] y1=A1×exp(t / B1)+C1

[0058] Where: y1 represents the surface stress of the square battery; A1 represents the initial response intensity of the surface stress. The larger its value, the greater the increase in the surface stress over time. exp(·) represents the exponential function. B1 represents the rate of increase of the surface stress over time. It determines how fast the surface stress increases over time. The smaller the value, the faster the change, and the larger the value, the smoother the change. C1 represents the baseline level of the initial surface stress, that is, the surface stress after the initial preload is applied. It represents the initial surface stress level of the system when time is zero or before it begins to change significantly. The larger the value, the higher the overall starting value.

[0059] The fitted relationship between the internal pressure and time of the square battery is expressed as follows:

[0060] y2=A2×exp(t / B2)+C2

[0061] Where: y2 represents the internal pressure of the square battery; A2 represents the initial response strength of the internal pressure; B2 represents the rate of increase of the internal pressure over time, which determines how fast the internal pressure rises over time. A smaller value indicates a faster change, and a larger value indicates a slower change; C2 represents the baseline level of the initial internal pressure.

[0062] Specifically, after a large number of tests by technicians in this field, it was found that both the internal pressure and the surface stress showed an exponential function trend. Therefore, this embodiment uses the origin software to use a conventional nonlinear exponential function to fit the surface stress and internal pressure of the square battery collected over a period of time to obtain their functional expressions.

[0063] In this embodiment, Figure 5 As shown in the figure, the surface stress data collected by the pressure film sensor and the internal pressure data measured by the threaded pressure sensor are used to draw curves respectively, and a nonlinear exponential function is used for fitting to obtain their functional expressions. The obtained functional expression of the surface stress changing with time is:

[0064] y1=0.0124exp(t / 77.9433)+0.1290

[0065] The obtained expression of the internal pressure changing with time is:

[0066] y2=0.0155exp(t / 89.3175)+0.0084

[0067] S5: Obtaining a functional relationship between the internal pressure and the surface stress of the square battery according to the fitted expression of the surface stress and time of the square battery and the fitted expression of the internal pressure and time of the square battery;

[0068] Preferably, the functional relationship between the internal pressure and the surface stress of the square battery is expressed as follows:

[0069]

[0070] Specifically, according to the fitted expression of the surface stress and time of the square battery and the fitted expression of the internal pressure and time of the square battery, by eliminating the intermediate variable t, the variation relationship between the internal pressure y2 and the surface stress y1 can be obtained.

[0071] S6: A thin film array pressure sensor is set on the square battery to be analyzed, and a preload force equal to the test preload force is applied; the square battery to be analyzed is heated to obtain the surface stress of the square battery to be analyzed, so as to obtain the internal pressure of the square battery to be interpreted based on the functional relationship between the internal pressure and the surface stress of the square battery, so as to realize the analysis of the internal pressure of the square battery.

[0072] Specifically, a preload force identical to the test preload force is applied to the square battery to be analyzed, and the same heating method as in step S3 is used to heat the square battery to be analyzed for a thermal runaway test, thereby obtaining data on the surface stress before the safety valve is opened, and thereby obtaining the change in the internal pressure of the square battery to be analyzed before the safety valve is opened.

[0073] In this embodiment, the internal pressure of the square battery to be analyzed before the safety valve opens is derived by measuring the data of the surface stress change before the safety valve opens, rather than directly measuring it. Therefore, the change in the internal pressure of the square battery can be obtained by non-destructive analysis.

[0074] Example:

[0075] Take a 50Ah lithium iron phosphate battery cell as an example. First, follow Figure 3 Set up the experimental setup. Use a heating plate to heat the battery until the battery safety valve opens. Use a thin-film array pressure sensor to obtain stress data on the battery surface, and a threaded pressure sensor to obtain internal pressure data. Then, fit the functional expressions of how the surface stress and internal pressure change over time. Then, mathematically derive the functional relationship between internal pressure and surface stress. This allows for non-destructive monitoring of internal pressure changes in the battery. Specifically, the following steps are performed:

[0076] (1) Place a heating plate with a heating power of 600W and the same size as the large surface of the battery on one side of the battery, and place 30 pressure sensors of the same size on the other side of the battery. Figure 2 As shown, the thin film array pressure sensor, battery and heating plate are wrapped with insulation cotton and fixed with a steel fixture. A test preload is applied to ensure that the thin film array pressure sensor and the battery surface are in close contact, as shown in FIG. Figure 3 As shown;

[0077] (2) Install the threaded pressure sensor on the pre-opened battery threaded hole. After installation is complete, seal it with high-temperature resistant thread fastening glue and let it stand for 24 hours. Figure 4 As shown;

[0078] (3) Connecting the signal outputs of surface stress and internal pressure changes to recorders to record surface stress and internal pressure change data;

[0079] (4) Based on the obtained data, the surface stress and internal pressure change curves are drawn respectively, such as Figure 5 As shown, nonlinear exponential functions are used to fit and obtain their respective functional expressions, and then the relationship between internal pressure and surface stress is derived mathematically, as shown in Figure 6 As shown;

[0080] (5) Conduct battery thermal runaway experiments under different working conditions, apply a test preload force to keep its surface stress consistent with the initial surface stress in step S1, obtain the change data of the surface stress before the safety valve opens, and substitute it into the relationship between the internal pressure and the surface pressure to obtain the internal pressure data.

[0081] The beneficial effects of this embodiment compared with the prior art are as follows:

[0082] 1. In this embodiment, when analyzing the internal pressure of a square battery to be analyzed, there is no need to open a hole in the battery shell, thereby avoiding damage to the sealing and structural integrity of the battery, and is suitable for non-destructive testing of the battery internal pressure.

[0083] 2. In this embodiment, the pressure film sensor directly applies a certain preload force and is arranged on the battery surface to measure stress. It supports array layout and can flexibly adapt to batteries of different sizes and shapes. It has wide applicability and versatility.

[0084] 3. In this embodiment, the pressure film sensor can adjust the data acquisition frequency to accurately capture the rapid changes in stress during battery expansion under different working conditions (such as 1C rate and 2C rate overcharging).

[0085] 4. This embodiment uses a nonlinear function to fit the variation curves of surface stress and internal pressure to obtain a functional relationship between the two, and then the magnitude of the internal pressure can be obtained through the surface stress value.

[0086] 5. The internal pressure obtained by indirect measurement of surface stress in this embodiment can also be used to provide early warning of battery thermal runaway and improve the safety of the battery system.

[0087] In summary, this embodiment combines a high-precision pressure film sensor with a mathematical function model to achieve non-invasive monitoring of the internal pressure of a lithium-ion battery, thereby providing an early warning before thermal runaway occurs and improving the safety of the battery system.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for analyzing the internal pressure of a square battery based on surface stress changes, characterized in that: The steps include: S1: A thin film array pressure sensor is placed on the surface of the square battery and a test preload is applied; S2: A pressure sensor with a threaded connector is fixedly connected to the square battery; S3: heating the square battery to obtain the surface stress of the square battery through the thin film array pressure sensor and the internal pressure of the square battery through the pressure sensor with a threaded connector before the battery safety valve is opened; thereby obtaining a curve showing the surface stress of the square battery and the internal pressure of the square battery varying with time; S4; using a nonlinear exponential function, fitting the curve of the surface stress of the square battery changing with time and the curve of the internal pressure of the square battery changing with time, respectively, to determine the expression relationship between the surface stress of the square battery and time after fitting and the expression relationship between the internal pressure of the square battery and time after fitting; S5: Obtaining a functional relationship between the internal pressure and the surface stress of the square battery according to the fitted expression of the surface stress and time of the square battery and the fitted expression of the internal pressure and time of the square battery; S6: A thin film array pressure sensor is set on the square battery to be analyzed, and a preload force equal to the test preload force is applied; the square battery to be analyzed is heated to obtain the surface stress of the square battery to be analyzed, so as to obtain the internal pressure of the square battery to be interpreted based on the functional relationship between the internal pressure and the surface stress of the square battery, so as to realize the analysis of the internal pressure of the square battery.

2. The method for analyzing the internal pressure of a square battery based on surface stress changes according to claim 1, characterized in that: In S5, the functional relationship between the internal pressure and the surface stress of the square battery is expressed as follows: Where: y2 represents the internal pressure of the square battery; C2 represents the baseline level of the initial internal pressure; A2 represents the initial response intensity of the internal pressure; y1 represents the surface stress of the square battery; A1 represents the initial response intensity of the surface stress; C1 represents the baseline level of the initial surface stress; B1 represents the rate of increase of the surface stress over time; B2 represents the rate of increase of the internal pressure over time.

3. The method for analyzing the internal pressure of a square battery based on surface stress changes according to claim 1, characterized in that: The surface of the square battery on which the thin film array pressure sensor is arranged is the surface with the largest area on the square battery.

4. The method for analyzing the internal pressure of a square battery based on surface stress changes according to claim 1, characterized in that: The method for fixing the pressure sensor with a threaded connector to the square battery is as follows: S201: Setting a threaded hole on the surface of the square battery; S202: Using the external thread provided on the pressure sensor with the threaded joint to connect with the threaded hole and tighten; S203: Seal the position where the threaded hole is connected and fastened.

5. The method for analyzing the internal pressure of a square battery based on surface stress changes according to claim 1, characterized in that: In S4, the fitted relationship between the surface stress and time of the square battery is expressed as follows: y1=A1×exp(t / B1)+C1 Where: y1 represents the surface stress of the square battery; A1 represents the initial response intensity of the surface stress; exp(·) represents the exponential function; B1 represents the rate at which the surface stress increases with time; C1 represents the baseline level of the initial surface stress; t represents time; The fitted relationship between the internal pressure and time of the square battery is expressed as follows: y2=A2×exp(t / B2)+C2 Where: y2 represents the internal pressure of the square battery; A2 represents the initial response intensity of the internal pressure; B2 represents the rate at which the internal pressure increases over time; C2 represents the baseline level of the initial internal pressure.

Images