Thermal characteristic parameter flow calorimetric test method for lithium ion battery based on entropy heat coefficient

Abstract

The application discloses a kind of lithium ion battery heat characteristic parameter flow heat test methods based on entropy heat coefficient.The present application utilizes lithium ion battery heat characteristic test calorimetric device as experimental device, by measuring the entropy heat coefficient of lithium ion battery in advance, control the internal heat production of charge-discharge process, the heat capacity of lithium ion battery is calculated and combined with lithium ion battery heat model to deduce the thermal conductivity of battery.Compared with traditional measurement method, the present application utilizes charge-discharge heat production to replace external heat source, realizes real in-situ excitation.The method reduces the interference of contact thermal resistance, environmental temperature and other external factors on the measurement of thermal conductivity and heat capacity of lithium ion battery, greatly improves the precision and applicability of measurement.At the same time, the method can measure the thermal conductivity and heat capacity of lithium ion battery simultaneously, significantly improves the test efficiency.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery thermal management technology, and more specifically relates to an AC calorimetric testing method for the thermal characteristic parameters of lithium-ion batteries based on the entropy coefficient. Background Technology

[0002] With the accelerated global energy structure transformation and the establishment of "dual-carbon" goals, electric vehicles, as the core carrier of green transportation, are facing critical development opportunities in their industrialization process. Lithium-ion batteries, with their advantages of high energy density, low self-discharge rate, and long cycle life, have become the mainstream technology for power batteries. In practical applications, operating temperature is a key factor affecting the lifespan, performance, and safety of lithium-ion batteries. High battery temperatures pose a risk of thermal runaway, while low temperatures reduce the battery's energy and power density. To ensure battery safety and optimal performance, the key lies in designing a reasonable battery thermal management system. Thermal simulation is an important tool for research on lithium battery thermal management and thermal safety. The accuracy of solving the heat transfer equations depends on the battery's basic thermal parameters, including heat generation power, specific heat capacity, thermal conductivity, and interfacial heat transfer coefficient.

[0003] Thermal capacity and thermal conductivity are key thermal characteristic parameters of lithium-ion battery thermal management systems. Existing technologies primarily employ mass-weighted and dynamic excitation methods to measure the thermal capacity of lithium-ion batteries. The mass-weighted method requires battery disassembly in an Ar atmosphere glove box, where the specific heat capacity of each component material (positive electrode, negative electrode, separator, etc.) is measured by DSC and then weighted by mass fraction. This method has two major limitations: destructive testing renders the samples unusable; and volatile components such as the electrolyte are difficult to measure accurately. The dynamic excitation method includes thermoelectric impedance spectroscopy (TIS) and electrothermal impedance spectroscopy (ETIS), using a lock-in amplifier to record the phase difference in surface temperature to invert the thermal capacity. However, recording the thermal response of a lithium-ion battery is a very time-consuming process, and the heat dissipation from the battery to the surrounding environment during testing introduces uncertainties.

[0004] The main methods for measuring the thermal conductivity of lithium-ion batteries include the component-weighted method, the steady-state method, and the unsteady-state method. The component-weighted method calculates the thermal conductivity based on the relevant parameters of each component. The steady-state method, based on Fourier's law, establishes a time-invariant temperature field within the battery under test, allowing heat to conduct in a one-dimensional manner. The temperature gradient and heat flow per unit area are then measured to determine the thermal conductivity of the lithium-ion battery. However, establishing a stable temperature gradient using this method is difficult and time-consuming. The unsteady-state method typically operates in an adiabatic environment, utilizing transient laser methods to measure the thermal conductivity of lithium-ion batteries. This method requires high-precision and high-sensitivity sensors and cannot perform in-situ measurements on the lithium-ion battery.

[0005] Existing research shows that the thermal capacity and thermal conductivity of lithium-ion batteries change with the state of charge. However, the aforementioned measurement methods generally employ offline measurements, and the results are only discrete values ​​under a specific state of charge. Furthermore, the two parameters are typically measured using different methods and testing equipment, inevitably increasing time and economic costs. Therefore, developing in-situ, rapid, and high-precision thermophysical property testing technologies has become a key issue in overcoming the bottlenecks in battery thermal management.

[0006] In short, determining the transient heat capacity and thermal conductivity during the charging and discharging process of lithium-ion batteries is very important in the fields of lithium-ion battery thermal characteristic research and thermal management system design. Summary of the Invention

[0007] To address the shortcomings of existing lithium-ion battery thermal capacity and thermal conductivity testing methods mentioned in the background section, such as offline measurement, complex equipment, and long processing times, which lead to deficiencies in thermal characteristic research and thermal management system design, this invention aims to provide an efficient, accurate, and widely applicable method for measuring the thermal conductivity and thermal capacity of lithium-ion batteries. By pre-measuring the entropy thermal coefficient of the lithium-ion battery and controlling the internal heat generation during charging and discharging, the thermal capacity of the lithium-ion battery is calculated, and the thermal conductivity is inferred by combining this with a lithium-ion battery thermal model.

[0008] To achieve the above objectives, this invention provides an AC calorimetric method for testing the thermal characteristic parameters of lithium-ion batteries based on the entropy coefficient, comprising the following steps:

[0009] Step 1: Obtain the entropy thermal coefficient of the lithium-ion battery under test;

[0010] Step 2: Select a heat-regulating block of the same size according to the geometric dimensions of the battery;

[0011] Step 3: Install the lithium-ion battery thermal characteristic testing calorimeter.

[0012] Step 4: Under constant temperature conditions, charge and discharge the battery, and record the battery temperature change and the current flowing through the battery during the experiment.

[0013] Step 5: Perform parametric modeling on the battery and record the temperature response data of the model under square wave current excitation at different frequencies;

[0014] Step 6: Based on the data collected in real time during the experiment, the heat capacity of the lithium-ion battery at different frequencies is calculated and used as the experimental basis for inversion calculation; by comparing the similarity between the experimental data and the simulation data, the thermal conductivity of the lithium-ion battery is obtained through inversion.

[0015] Based on the above technical solution, the present invention has the following beneficial effects:

[0016] (1) This invention controls the internal heat generation during the charging and discharging process and uses a thermal model to inversely solve for the thermal conductivity and heat capacity of lithium-ion batteries. This method reduces the interference of external factors such as contact thermal resistance and ambient temperature on the measurement of the thermal conductivity and heat capacity of lithium-ion batteries, greatly improving the accuracy and applicability of the measurement. In addition, this invention can be used to study the changes in the thermal properties of lithium-ion batteries under different SOC, temperature and current.

[0017] (2) This invention is applicable to full-battery level measurements, without the need to disassemble the battery, and is closer to actual working conditions.

[0018] (3) The present invention can simultaneously measure the thermal conductivity and heat capacity of lithium-ion batteries, which significantly improves the testing efficiency and reduces economic costs.

[0019] (4) This invention uses internal heat generation to infer thermophysical parameters, avoiding external interference, and has the advantage of in-situ measurement, making it suitable for multi-condition research.

[0020] (5) The present invention can simultaneously measure the anisotropic thermal conductivity of lithium-ion batteries. Compared with the traditional method of measuring anisotropic thermal conductivity, which requires adjustment of the experimental device, the test efficiency is improved. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a calorimeter for testing the thermal characteristics of lithium-ion batteries according to an embodiment of this application.

[0022] Figure 2 This is a schematic diagram of the equivalent circuit model of a lithium-ion battery according to an embodiment of this application;

[0023] Figure 3 This is a schematic diagram of a lithium-ion battery simulation model according to an embodiment of this application;

[0024] Figure 4 This is a flowchart illustrating the thermal parameter inversion process in an embodiment of this application.

[0025] Figure 5 This is a schematic diagram of the frequency-temperature response fitting curve of an embodiment of this application. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] like Figures 1-4As shown, in order to accurately obtain the thermal characteristic parameters of lithium-ion batteries during the charging and discharging process, this application proposes an AC calorimetric testing method for the thermal characteristic parameters of lithium-ion batteries based on the entropy thermal coefficient, which significantly improves the testing efficiency of thermal characteristic parameters. By combining the heat generation characteristics of lithium-ion batteries during charging and discharging, the model is made to better fit the actual working conditions, greatly enriching the application of thermal parameter measurement in the design of lithium-ion battery thermal management systems.

[0028] The method first obtains the entropic thermal coefficient of the lithium-ion battery using the open-circuit voltage method or frequency domain analysis. Under isothermal conditions, the lithium-ion battery is charged and discharged using a square wave current of a certain frequency. Simultaneously, a temperature sensor measures the surface temperature field of the lithium-ion battery. The heat capacity of the lithium-ion battery is calculated by the temperature response amplitude at different frequencies, and the anisotropic thermal conductivity of the battery is inverted using a lithium-ion battery simulation model. This method achieves the measurement of the heat capacity and thermal conductivity of the lithium-ion battery under conditions where the heat generated by charging and discharging is used as the heat source.

[0029] The specific steps of this application embodiment are as follows:

[0030] Step 1: Obtain the entropy thermal coefficient of the lithium-ion battery under test using the open-circuit voltage method or frequency domain analysis method.

[0031] Step 2: Select a uniform heating block of the same size according to the geometric dimensions of the selected battery to be tested.

[0032] Step 3: Install the calorimeter for testing the thermal characteristics of lithium-ion batteries.

[0033] Step 4: Use an external oil bath to maintain a constant heat sink temperature. Once the temperature stabilizes, set the external charging / discharging equipment to pulse charging / discharging mode and start it. Record the temperature change of the lithium-ion battery and the current flowing through the battery during this process.

[0034] Step 5: Perform parametric modeling of the lithium-ion battery. By combining the lumped parameter thermal model (for rapid heat generation) and the solid heat transfer model (for accurate temperature distribution), record the temperature response data of the model under square wave current excitation at different frequencies.

[0035] Step 6: By collecting the temperature responses of various surfaces of the lithium-ion battery in real time during the experiment, the heat capacity of the lithium-ion battery at different frequencies can be calculated. Simultaneously, this temperature response data can serve as the experimental basis for inversion calculations. By comparing the similarity between the experimental data and the simulation data, the thermal conductivity of the lithium-ion battery sample can be obtained through inversion.

[0036] Furthermore, step 1 specifically includes:

[0037] Taking the open-circuit voltage method as an example, the lithium-ion battery is placed between two constant-temperature heat sinks in the calorimeter's calorimeter chamber, a temperature sensor is fixed on the surface, and the positive and negative terminals are connected to the E34470A7 half-digit multimeter via wires.

[0038] The lithium-ion battery was adjusted to the desired state of charge (SOC) for testing, and the temperature of the constant-temperature heat sink was changed every 1 hour. During this period, the open-circuit voltage and surface temperature of the lithium-ion battery were measured in real time.

[0039] After one cycle of temperature change, at an ambient temperature of 20°C, 10% of the rated capacity of the lithium-ion battery was discharged at 0.1C, and then left to stand for 12 hours. The change of open-circuit voltage with temperature was then measured.

[0040] Using the open-circuit voltage method, measurements were taken starting from a SOC of 90% and continuing until the SOC reached 10%. By recording the changes in lithium-ion battery voltage with temperature at different SOC states, the entropy-thermal coefficient of the lithium-ion battery was obtained.

[0041] Furthermore, step 2 specifically includes:

[0042] Based on the geometry of the selected lithium-ion battery to be tested, select a uniform heating block of appropriate size, and record the thickness and surface area of ​​the lithium-ion battery.

[0043] Determine the charge and discharge parameters based on the specifications of the battery under test, and select a battery with an appropriate wire diameter for charging and discharging.

[0044] The wires are connected to the outside via a four-wire system using a high-airtightness aviation connector.

[0045] Furthermore, step 3 specifically includes:

[0046] An oil bath pipe is used to connect the external constant temperature oil bath equipment to the constant temperature heat sink inside the calorimetric chamber in order to maintain a constant temperature boundary.

[0047] An inlet valve and a gas flow meter are designed and installed on the side wall of the calorimeter for testing the thermal characteristics of lithium-ion batteries, and an outlet valve and a pressure relief valve are installed on the cover. The inlet valve is used to introduce dry gas, the gas flow meter is used to control the flow rate of the introduced gas, the outlet valve is used to discharge air, and the pressure relief valve is used to prevent danger caused by excessive gas pressure in the calorimeter chamber.

[0048] The battery charging and discharging leads are connected to the outside via a four-wire system using a high-airtightness aviation connector.

[0049] Furthermore, the method for installing the calorimetric cavity is as follows:

[0050] During the lithium-ion battery charge-discharge and thermal characteristic measurement experiment, the components are installed between the upper and lower heat sinks of the isothermal calorimeter chamber in the following order from top to bottom: heat distribution block, battery under test, heat distribution block.

[0051] The positive and negative terminals of the battery under test are connected to an external battery charging and discharging device via wires and electrical connectors on the calorimeter cavity.

[0052] A thermistor temperature sensor is installed simultaneously in the groove of the upper and lower heat equalizing blocks close to the battery side and on the side of the battery under test, and it must fit well into the groove.

[0053] Insulating cotton is laid around the calorimeter cavity, and the calorimeter cavity is sealed after the battery is installed.

[0054] Installation results are as follows: Figure 1 As shown, the lithium-ion battery thermal characteristic testing calorimeter includes: a battery under test 1, insulation cotton 2, a temperature sensor 3, a heat equalization block 4, a charge / discharge cable 5, a constant temperature oil bath 6, a heat sink 7, an oil bath pipe 8, a high-airtightness aviation connector 9, a gas flow meter 10, an inlet valve 11, an outlet valve 12, and a pressure relief valve 13. The external constant temperature oil bath 6 is connected to the internal constant temperature heat sink 7 of the calorimeter chamber via the oil bath pipe 8 to maintain a constant temperature boundary. The heat equalization block 4 is used to even out the battery temperature; its size is consistent with that of the battery under test 1, and it has grooves for installing the temperature sensor 3. The side wall is designed with an inlet valve 11 and a gas flow meter 10, and the cover is equipped with an outlet valve 12 and a pressure relief valve 13. The inlet valve 11 is used for the entry of dry gas, the gas flow meter 10 is used for controlling the flow rate of the introduced gas, the outlet valve 12 is used for the discharge of air, and the pressure relief valve 13 is used to prevent danger caused by excessive gas pressure inside the calorimeter chamber.

[0055] Furthermore, step 4 specifically includes:

[0056] Step 4.1: During the experiment, the external circulation of the oil bath is turned on so that silicone oil at a certain temperature is pumped into the heat sink, thereby controlling the heat sink at a certain constant temperature value.

[0057] If the required isothermal calorimetric temperature for the battery under test is lower than room temperature, the calorimetric chamber needs to be purged with a drying gas. During purging, first close the calorimetric chamber, connect the drying gas from an external nitrogen cylinder to the inlet valve on the calorimetric chamber via a pipeline, and then sequentially open the inlet and outlet valves on the calorimetric chamber wall. The purging time can be adjusted according to the gas flow rate displayed by the gas flow meter on the calorimetric chamber wall. During the gas purging process, if the pressure inside the chamber becomes too high, open the pressure relief valve installed on the calorimetric chamber.

[0058] After the replacement is complete, close the intake valve and exhaust valve.

[0059] Step 4.2: After verifying that the wiring connections are correct, start the device, set the oil bath temperature and configure the oil bath to external circulation temperature control mode, keeping the heat sink temperature at the constant temperature point required for the experiment. Then start the battery charging and discharging equipment, set the charging and discharging parameters of the lithium-ion battery under different operating conditions, and use the host computer to collect and record real-time parameters such as the temperature of the battery under test during the charging and discharging process.

[0060] Furthermore, step 5 specifically includes:

[0061] A lumped thermal parameter model of a lithium-ion battery was established using COMSOL multiphysics simulation software. Square wave currents of different frequencies were applied to the lithium-ion battery simulation model, and the changes in the surface temperature field of the model were observed and recorded.

[0062] Furthermore, the schematic diagram of the lithium-ion battery simulation model is as follows: Figure 2 As shown, the model fully presents the main structural components of the battery and the spatial arrangement of each key component. As shown in the figure, the battery structure, from top to bottom, includes: a top cover 14, serving as a sealed cover and integrating positive and negative electrode output terminals; electrodes 15, containing positive and negative electrode active material layers; connecting pieces 16, enabling conductive connections between the electrodes and external circuits; current collectors 17 (aluminum foil for the positive electrode and copper foil for the negative electrode), used to collect and conduct current from the electrode active materials; a core 18, the core energy storage unit formed by stacking and winding positive and negative electrode sheets and a separator; and a casing 19, made of stainless steel, providing mechanical support and ensuring battery sealing. This schematic diagram clearly shows the assembly relationship between the components, providing an accurate geometric modeling foundation for subsequent multiphysics simulations of the battery.

[0063] Specifically, in this embodiment, COMSOL Multiphysics is used for simulation software. The "Battery and Fuel Cell" module is selected, and a "Lumped Battery Interface" is added to simplify electrochemical-thermal coupling calculations. A "Solid Heat Transfer" interface is added to simulate the battery temperature field distribution. A simplified geometry is used to represent the battery cell, with dimensions set according to the actual cell size (100mm × 60mm × 10mm). A "Rectangular Wave" function is created in "Global Definition" to define the square wave signal. In "Multiphysics," the "Lumped Battery Heat Source" is enabled to automatically calculate Joule heat and reversible heat, coupling with the "Solid Heat Transfer" interface.

[0064] Next, parameterized variables were set, defining the parameters to be studied (such as thermal conductivity or heat capacity), and multiple simulations were automatically run using "parametric scan". Temperature field monitoring used domain point probes, placing probes at the center and edges of the battery surface to record temperature changes over time. Simultaneously, a surface temperature contour map was set, generating a transient temperature distribution animation in the "Results" section to observe hotspot formation. The temperature-time curves were then exported as a CSV file for subsequent analysis.

[0065] Furthermore, step 6 specifically includes:

[0066] Steady-state heat generation power calculation: Based on the real-time data collected during the experiment and the entropy coefficient, the heat generation power of the battery after reaching steady state is calculated.

[0067] Fast Fourier Transform (FFT) analysis: Converts the time-domain temperature signal and heat generation power to the frequency domain and extracts the amplitude-frequency relationship;

[0068] Heat capacity fitting: The heat capacity of the lithium-ion battery is obtained by linearly fitting the quotient of the heat generation power and the temperature change amplitude at different frequencies.

[0069] Anisotropic thermal conductivity inversion: By measuring temperature changes and combining the heat balance equation, thermophysical parameters are inverted to establish a heat transfer model; the experimental data and theoretical model are fitted to solve for the optimal thermal conductivity, and the inverted anisotropic thermal conductivity is obtained.

[0070] Furthermore, the overall calculation process is as follows:

[0071] The heat capacity of a lithium-ion battery can be calculated using experimentally measured temperature data and other known parameters (entropy coefficient, current, geometric dimensions, material density, etc.).

[0072] Specifically, lithium-ion batteries generate or absorb heat during charging and discharging. This heat generation during charging and discharging is generally categorized into irreversible heat, reversible heat, internal side reaction heat, and mixing heat. The side reaction heat caused by battery aging is released slowly and can be ignored. In the electrochemical system, the mass transport characteristics are good, and the concentration gradient is limited; therefore, the mixing heat caused by changes in the concentration of components within the lithium-ion battery can be neglected. Thus, a simplified heat generation model for lithium-ion batteries is shown below:

[0073]

[0074] in: This provides heat generation power for lithium-ion batteries. The irreversible heat generation power of lithium-ion batteries; I represents the reversible heat generation power of the lithium-ion battery; R represents the current flowing through the lithium-ion battery; and R represents the internal resistance of the lithium-ion battery. This represents the open-circuit voltage of the lithium-ion battery; T represents the absolute temperature of the lithium-ion battery. This represents the entropy thermal coefficient of a lithium-ion battery.

[0075] When the state of charge (SOC) of a lithium-ion battery varies within a very small range, its internal resistance can be considered constant. Under isothermal conditions, using a square wave current of a certain frequency for charging and discharging, irreversible thermal expansion occurs in steady state. It will be a constant, reversible heat. It is a square wave with the same frequency as the input current. Using the square wave current as the charging and discharging current, the waveform frequency of the reversible heat is the same as the frequency of the input current. Given the entropy thermal coefficient of the lithium-ion battery, the heat generation power of the lithium-ion battery after reaching steady state can be calculated.

[0076] The schematic diagram of the equivalent circuit model of the lithium-ion battery is as follows: Figure 3 As shown, where, For the surface temperature of lithium-ion batteries, For ambient temperature, q b To generate heat for lithium-ion batteries, For lithium-ion battery heat capacity, For lithium-ion battery thermal resistance, This refers to the thermal resistance between the environment and the battery.

[0077] For lithium-ion batteries, the thermal energy change consists of two parts: one is the heat power generated or absorbed during charging and discharging, and the other is the heat transfer power through heat conduction with the environment. The equation for the battery thermal energy change is shown below:

[0078]

[0079] Perform a Fourier transform on both sides of the equation:

[0080]

[0081] Let K = 1 / :

[0082]

[0083] Take the mold from both sides:

[0084]

[0085] The heat capacity of a lithium-ion battery can be obtained by applying square wave currents of different frequencies and performing linear fitting on the results.

[0086] By combining a lumped thermal parameter model, the anisotropic thermal conductivity of the target material can be inverted. Using the known entropy thermal coefficient and the charge / discharge behavior of lithium-ion batteries, the reversible heat generation of the lithium-ion battery can be estimated and combined with the calculated heat capacity and the inverted thermal conductivity. The inverse heat transfer problem can be viewed as an optimization problem; most methods for solving inverse problems are based on least squares, gradient descent, and genetic algorithms to minimize the difference between experimental and computational data. During inversion, the anisotropic thermal conductivity of the material can be inferred by measuring temperature changes in different directions.

[0087] The temperature rise of a lithium-ion battery is determined by the heat generation rate and heat dissipation conditions. By measuring the temperature change and combining it with the heat balance equation, the thermophysical parameters can be deduced. The heat transfer model is shown in the following equation:

[0088]

[0089] Initial conditions:

[0090]

[0091] The boundary conditions are:

[0092]

[0093]

[0094]

[0095] in, This refers to the density of lithium-ion batteries; The heat capacity of a lithium-ion battery; The temperature of the lithium-ion battery is t; time is t. This provides heat generation power for lithium-ion batteries. , , The thermal conductivity is anisotropic. The ambient temperature is h; h is the heat transfer coefficient. , , These are the length, width, and thickness of the battery cell, respectively.

[0096] Furthermore, applying square wave currents of different frequencies to a lithium-ion battery results in varying temperature responses. Under low-frequency excitation, thermal conductivity has a more significant impact on temperature changes, while under high-frequency excitation, heat capacity has a more pronounced effect on the temperature response.

[0097] To achieve the inversion, optimization algorithms are typically used to fit the experimental data and theoretical models to solve for the optimal thermal conductivity. , , As shown in the following formula:

[0098]

[0099] in, It is the temperature response calculated based on a theoretical model; It is the temperature response measured experimentally; These are the different frequencies used in the experiment; by optimizing this objective function, the anisotropic thermal conductivity can be obtained.

[0100] The flowchart of thermal parameter inversion described in this application is as follows: Figure 4 As shown, in this inversion method, the objective function is the root mean square error (RMSE) between the experimental data and the simulation calculation data within the region of interest. The thermal conductivity is determined through an iterative optimization method. , , The optimal value is found to minimize the root mean square error (RMSE) between the simulation and experimental data. The first step involves initially setting the parameters based on empirical or theoretical estimations. , , The range of values ​​for which parameters are taken provides a search space for subsequent optimization; the second step is to optimize the parameters step by step, fixing... and Adjust in a certain step size Calculate the RMSE of simulation and experimental data, and select the value that minimizes the RMSE. The value is taken as the current optimal solution, and subsequent optimizations are performed using the same method. and The value of the parameter is iterated through multiple iterations to gradually approach the optimal solution for each parameter. The third step checks if the current parameter range is sufficiently precise. If the range is still too wide or the RMSE has not converged, the process returns to the second step to continue optimization. If the range has converged to a reasonable interval, the process enters the global traversal phase. The fourth step, within the optimized parameter range, uses a smaller step size to further refine the parameter calculation. , , We iterate through all possible combinations, calculate the parameter combination that minimizes the RMSE, select the combination that minimizes the RMSE as the final inversion result, and finally output the optimal result. , , Complete the inversion process.

[0101] Figure 5 This is an embodiment of the present application. - Temperature response amplitude fitting curve. The square root of the slope of the curve in the figure is the estimated heat capacity value. The estimated heat capacity value of the lithium-ion battery in this embodiment is 949.246 J / K, with an error of 1.5% compared with the standard value of 938 J / K, demonstrating good measurement accuracy. Furthermore, the measurement time is reduced to 1 / 3 of the traditional method, indicating that this method improves the measurement efficiency of lithium-ion battery heat capacity and greatly enriches the application of isothermal calorimetry in the field of battery research.

[0102] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for AC calorimetry testing of thermal characteristic parameters of lithium-ion batteries based on entropy coefficient, characterized in that, Includes the following steps: Step 1: Obtain the entropy thermal coefficient of the lithium-ion battery under test; Step 2: Select a heat-regulating block of the same size according to the geometric dimensions of the battery; Step 3: Install the lithium-ion battery thermal characteristic testing calorimeter. Step 4: Under constant temperature conditions, charge and discharge the battery, and record the battery temperature change and the current flowing through the battery during the experiment. Step 5: Perform parametric modeling on the battery and record the temperature response data of the model under square wave current excitation at different frequencies; Step 6: Based on the data collected in real time during the experiment, the heat capacity of the lithium-ion battery at different frequencies is calculated and used as the experimental basis for inversion calculation; by comparing the similarity between the experimental data and the simulation data, the thermal conductivity of the lithium-ion battery is obtained through inversion. Step 3 specifically involves: An oil bath pipe is used to connect the external constant temperature oil bath equipment to the constant temperature heat sink inside the calorimetric chamber in order to maintain a constant temperature boundary. An inlet valve and a gas flow meter are designed and installed on the side wall of the calorimeter for testing the thermal characteristics of lithium-ion batteries, and an outlet valve and a pressure relief valve are installed on the cover. The inlet valve is used to introduce dry gas, the gas flow meter is used to control the flow rate of the introduced gas, the outlet valve is used to discharge air, and the pressure relief valve is used to prevent danger caused by excessive gas pressure in the calorimeter chamber. The battery charging and discharging leads are connected to the outside via a high-airtightness aviation plug in a four-wire configuration; The calorimetric cavity is installed as follows: The components are installed between the upper and lower heat sinks of the isothermal calorimeter chamber in the following order from top to bottom: heat homogenizing block, battery under test, heat homogenizing block. The positive and negative terminals of the battery under test are connected to an external battery charging and discharging device via wires and electrical connectors on the calorimetric cavity. Temperature sensors are installed simultaneously in the grooves of the upper and lower heat equalizing blocks close to the battery side and on the side of the battery under test, and are fitted into the grooves. Insulation cotton is laid around the calorimeter cavity, and the calorimeter cavity is sealed after the battery is installed. Step 6 specifically involves: Steady-state heat generation power calculation: Based on the real-time data collected during the experiment and the entropy coefficient, the heat generation power of the battery after reaching steady state is calculated. Fast Fourier Transform (FFT) analysis: Converts the time-domain temperature signal and heat generation power to the frequency domain and extracts the amplitude-frequency relationship; Heat capacity fitting: The heat capacity of the lithium-ion battery is obtained by linearly fitting the quotient of the heat generation power and the temperature change amplitude at different frequencies. Anisotropic thermal conductivity inversion: By measuring temperature changes and combining the heat balance equation, thermophysical parameters are inverted to establish a heat transfer model; the experimental data and theoretical model are fitted to solve for the optimal thermal conductivity, and the inverted anisotropic thermal conductivity is obtained.

2. The method according to claim 1, characterized in that, The entropy thermal coefficient of the lithium-ion battery is obtained using the open-circuit voltage method or the frequency domain analysis method.

3. The method according to claim 1, characterized in that, Step 2 specifically involves: Based on the geometry of the selected lithium-ion battery to be tested, select a uniform heating block of appropriate size, and record the thickness and surface area of ​​the battery. Determine the charging and discharging parameters according to the battery specifications, select appropriate wire diameter battery charging and discharging wires, and connect them to the outside in a four-wire manner using a high-airtightness aviation plug.

4. The method according to claim 1, characterized in that, Step 4 specifically involves: Step 4.1: During the experiment, the external circulation of the oil bath is turned on so that silicone oil at a certain temperature is pumped into the heat sink, thereby controlling the heat sink at a certain constant temperature value. Step 4.2: After verifying that the circuit connection is correct, start the device, set the oil bath temperature and set the oil bath to external circulation temperature control mode, and control the heat sink temperature at the constant temperature point required for the experiment; start the battery charging and discharging equipment, set the charging and discharging parameters of the lithium-ion battery under different operating conditions, and use the host computer to collect and record the battery temperature change and the current flowing through the battery during the charging and discharging process.

5. The method according to claim 4, characterized in that, Step 4.1 further includes: If the required isothermal calorimetric temperature of the battery is lower than room temperature, the calorimetric chamber is replaced with a dry gas. During replacement, first close the calorimeter chamber, connect the dry gas in the external nitrogen cylinder to the inlet valve on the calorimeter chamber through the pipeline, and then open the inlet valve and outlet valve on the wall of the calorimeter chamber in sequence. The drying gas replacement time is adjusted based on the gas flow rate displayed by the gas flow meter on the calorimeter wall. During the gas replacement process, if the gas pressure inside the chamber is too high, the pressure relief valve installed on the calorimetric chamber will be opened. After the replacement is complete, close the intake valve and exhaust valve.

6. The method according to claim 1, characterized in that, In step 5, the model structure, from top to bottom, includes: The top cover serves as a sealed cover for the battery and integrates positive and negative output terminals; Electrode, comprising positive and negative electrode active material layers; Connecting tabs are used to achieve conductive connections between electrodes and external circuits; Current collectors are used to collect and conduct current in the active material of electrodes; The core energy storage unit is composed of positive and negative electrode sheets and a separator layered and wound together. The casing provides mechanical support and ensures the battery is sealed.

7. The method according to claim 1, characterized in that, In the anisotropic thermal conductivity inversion step, the objective function is the root mean square error between the experimental data and the simulation calculation data in the region of interest. The optimal value of the thermal conductivity is determined by an iterative optimization method, so as to minimize the root mean square error between the simulation data and the experimental data.

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