Method for measuring normal heat conduction coefficient of soft package battery

Abstract

The invention discloses a method for measuring a normal heat conduction coefficient of a soft package battery, and relates to the technical field of thermophysical parameter detection. The battery in an approximately adiabatic environment is heated at constant power through the flexible heater, and heat flux and temperature data of a heating surface and a cooling surface of the battery are respectively monitored by the heat flux sensor and the thermocouple; and after the temperature field of the battery reaches the quasi-steady state, calculating the normal heat conduction coefficient of the battery based on the established quasi-steady state heat conduction theoretical model. The method is easy and convenient to operate, the normal heat conduction coefficient can be represented only by monitoring the heat flux and temperature in the battery heating process, the measurement period is short, measurement is completed within 20 minutes, precision is high, cost is low, and key support can be provided for design of a battery heat management system.

Description

Technical Field

[0001] This invention relates to the field of thermophysical parameter detection technology, and in particular to a method for measuring the normal thermal conductivity of a pouch cell. Background Technology

[0002] As the core power source for electric vehicles, lithium-ion batteries rely heavily on their thermal properties for analyzing battery thermal behavior and designing battery thermal management systems. Large-sized pouch lithium-ion batteries are widely used due to their high energy density and lack of memory effect during charging and discharging. However, manufacturers rarely provide their thermal properties in their datasheets, and the impact of operating temperature on battery thermal conductivity has not been fully studied. Therefore, precise and efficient measurement methods are urgently needed to characterize their normal thermal conductivity.

[0003] Existing methods for measuring the thermal conductivity of lithium-ion batteries have several drawbacks: Xenon flash (XFT) technology requires expensive and bulky instruments that are not widely available; some methods based on the principle of heat conduction require extensive numerical simulations, which are labor-intensive and do not consider the effect of operating temperature on thermal conductivity. Furthermore, existing methods are mostly designed for cylindrical and prismatic hard-shell batteries, and there are few reports on dedicated measurement methods for large pouch lithium-ion batteries, making it difficult to meet the practical needs of battery thermal management system design.

[0004] To address the aforementioned issues, a method for measuring the normal thermal conductivity of pouch batteries is proposed to overcome the shortcomings of existing battery normal thermal conductivity measurement techniques. Summary of the Invention

[0005] In view of this, the present invention provides a method for measuring the normal thermal conductivity of a pouch battery, in order to solve the problems of high measurement cost, long cycle and unsuitability for large-size samples in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for measuring the normal thermal conductivity of a pouch battery includes the following steps: S1. Sample pretreatment: Wrap the soft-pack battery with thermal insulation material, and then place it in a constant temperature environment, taking the small side of the battery as the heat insulation surface. S2. Test setup: A flexible heater is attached to the main surface of the battery. A heat flow sensor and a thermocouple are placed at the center of the heating and cooling surfaces, respectively. The heat flow sensor is used to collect heat flux, and the thermocouple is used to collect temperature. S3. Creating a constant temperature environment: Place the prepared test device in the thermostat chamber and set the initial temperature of the chamber so that the sample temperature is consistent with the chamber temperature. S4. Thermal excitation loading and data acquisition: Power the flexible heater with a regulated power supply to heat the main surface of the sample, i.e. the heating surface, at a constant power. At the same time, record the heating time, heat flux and temperature change of the heating and cooling surfaces until the set upper limit temperature threshold is reached and then stop heating. S5. Quasi-steady state determination: Based on the collected temperature data, calculate the temperature difference between the battery heating surface and the cooling surface. When the temperature difference tends to be constant within a preset time, it is determined that the battery temperature field has reached a quasi-steady state. S6. Calculation of Normal Thermal Conductivity: Based on Fourier's law of thermal conductivity and the law of conservation of energy, a theoretical model for the normal thermal conductivity of a battery is derived. Substituting the temperature difference, heat flux, and battery size under quasi-steady-state conditions into the model, the normal thermal conductivity is calculated as follows:

[0007] In the formula: For normal thermal conductivity, L For battery thickness, For a constant heating power density of the heating surface, For the heat flux of the heating surface, For the heat flux of the cooling surface, A The surface area of ​​the heating film. The temperature of the heating surface. This refers to the temperature of the cooling surface.

[0008] Optionally, in the above method, the insulation material in contact with the battery in S1 is an aerogel felt, which is then covered with expanded polystyrene insulation material, and the thickness of the aerogel felt is 20 mm.

[0009] Optionally, in the above method, the sample in S1 is a pouch lithium-ion battery, including ternary pouch lithium-ion batteries, lithium iron phosphate pouch lithium-ion batteries, and other types of square pouch batteries.

[0010] In the above method, optionally, the accuracy of the heat flow sensor in S2 is ±2%, the thermocouple is a T-type thermocouple with a wire diameter of 0.26 mm and an accuracy of ±0.4%, and the size of the flexible heater matches the size of the main surface of the sample.

[0011] Optionally, in the above method, the input power of the flexible heater in S4 satisfies that the temperature difference between the heating surface and the cooling surface of the sample is 5~15 ℃, and the upper limit temperature threshold is 65 ℃. The heating power in S4 is constant, and the actual output power of the flexible heater is monitored and recorded in real time through a DC power supply.

[0012] Optionally, in the above method, the preset time in S5 is 84 seconds. When the temperature difference tends to be constant after 84 seconds, the current time node is determined as the quasi-steady-state starting point of the temperature field, and the temperature and heat flux data after the current node are used for calculation.

[0013] Optionally, the above method also includes S7, method verification: using a quartz glass block with known thermal parameters as a standard sample, and measuring according to S1~S6. When the measurement error of the thermal conductivity of the standard sample is ≤6.5%, the measurement method is deemed valid.

[0014] Optionally, the above method can measure the normal thermal conductivity of the sample in the temperature range of -20 to 60 ℃, and characterize the normal thermal conductivity of the sample at different working temperatures by changing the initial temperature.

[0015] As can be seen from the above technical solution, compared with the prior art, the present invention provides a method for measuring the normal thermal conductivity of a soft-pack battery, which has the following beneficial effects: Fast and convenient: Compared with methods that require a lot of simulation or complex equipment, this invention is based on a clear theoretical model and can measure the normal thermal conductivity of the battery in a short time (e.g., within 20 minutes) simply by monitoring the heat flux and temperature of the heated battery surface. Low cost: The only equipment required is a heat flow meter, thermocouple, DC power supply and heating film, without the need for expensive dedicated thermophysical property analyzers; Wide applicability: It is especially suitable for large-sized flat samples, such as large soft-pack lithium-ion batteries, overcoming the size limitations of traditional precision instrument test chambers; High accuracy: By monitoring the heat loss of the two main surfaces of the sample in real time and subtracting it from the total heating power, the error caused by environmental heat loss is effectively corrected, thus improving the measurement accuracy; Functional expansion: By using a constant temperature environment chamber, the normal thermal conductivity of a sample at different temperatures can be easily measured, providing an effective means to study its temperature change characteristics. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0017] Figure 1 This is a flowchart of a method for measuring the normal thermal conductivity of a soft-pack battery disclosed in this invention. Figure 2 This is a schematic diagram showing the arrangement of the sample and testing equipment disclosed in this invention; Figure 3 The figures are the temperature and heat flux change curves during the heating process of the NCM soft-pack battery disclosed in this invention, where 3(a) is the temperature-time curve and 3(b) is the heat flux-time curve. Figure 4 The temperature difference and heating rate curves of the NCM soft-pack battery disclosed in this invention are shown, where 4(a) is the temperature difference-time curve and 4(b) is the heating rate-time curve. Figure 5 The figures show the relationship curves between the normal thermal conductivity of the soft-pack battery and the operating temperature disclosed in this invention, where 5(a) is the relationship curve between the normal thermal conductivity of the NCM battery and the operating temperature, and 5(b) is the relationship curve between the normal thermal conductivity of the LFP battery and the operating temperature. Detailed Implementation

[0018] 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.

[0019] In this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0020] See Figure 1 As shown, this invention discloses a method for measuring the normal thermal conductivity of a pouch battery, comprising the following steps: S1. Sample pretreatment: Wrap the soft-pack battery with thermal insulation material, and then place it in a constant temperature environment, taking the small side of the battery as the heat insulation surface. S2. Test setup: A flexible heater is attached to the main surface of the battery. A heat flow sensor and a thermocouple are placed at the center of the heating and cooling surfaces, respectively. The heat flow sensor is used to collect heat flux, and the thermocouple is used to collect temperature. S3. Creating a constant temperature environment: Place the prepared test device in the thermostat chamber and set the initial temperature of the chamber so that the sample temperature is consistent with the chamber temperature. S4. Thermal excitation loading and data acquisition: Power the flexible heater with a regulated power supply to heat the main surface of the sample, i.e. the heating surface, at a constant power. At the same time, record the heating time, heat flux and temperature change of the heating and cooling surfaces until the set upper limit temperature threshold is reached and then stop heating. S5. Quasi-steady state determination: Based on the collected temperature data, calculate the temperature difference between the battery heating surface and the cooling surface. When the temperature difference tends to be constant within a preset time, it is determined that the battery temperature field has reached a quasi-steady state. S6. Calculation of Normal Thermal Conductivity: Based on Fourier's law of thermal conductivity and the law of conservation of energy, a theoretical model for the normal thermal conductivity of a battery is derived. Substituting the temperature difference, heat flux, and battery size under quasi-steady-state conditions into the model, the normal thermal conductivity is calculated as follows:

[0021] In the formula: For normal thermal conductivity, L For battery thickness, For a constant heating power density of the heating surface, For the heat flux of the heating surface, For the heat flux of the cooling surface, A The surface area of ​​the heating film. The temperature of the heating surface. This refers to the temperature of the cooling surface.

[0022] Furthermore, the insulation material in S1 that comes into contact with the battery is an aerogel felt, which is then covered with expanded polystyrene insulation material. The thickness of the aerogel felt is 20 mm.

[0023] Furthermore, the sample in S1 is a large-size pouch lithium-ion battery, including ternary pouch lithium-ion batteries, lithium iron phosphate pouch lithium-ion batteries, and other types of square pouch batteries.

[0024] Furthermore, the S2 heat flow sensor has an accuracy of ±2%, the thermocouple is a T-type thermocouple with a wire diameter of 0.26 mm and an accuracy of ±0.4%, and the size of the flexible heater matches the size of the main surface of the sample.

[0025] Furthermore, the input power of the flexible heater in S4 satisfies the requirement that the temperature difference between the heating and cooling surfaces of the sample is 5~15℃, and the upper limit temperature threshold is 65℃. The heating power in S4 is constant, and the actual output power of the flexible heater is monitored and recorded in real time through a DC power supply.

[0026] Furthermore, the preset time in S5 is 84 seconds. When the temperature difference tends to be constant after 84 seconds, the current time node is determined as the quasi-steady-state starting point of the temperature field, and the temperature and heat flux data after the current node are used for calculation.

[0027] Furthermore, it also includes S7, the method verification step: using a quartz glass block with known thermal parameters as a standard sample, and measuring according to S1~S6. When the measurement error of the thermal conductivity of the standard sample is ≤6.5%, the measurement method is deemed valid.

[0028] Furthermore, the normal thermal conductivity of the sample can be measured in the temperature range of -20 to 60 ℃. By changing the initial temperature of the thermostat cavity, the normal thermal conductivity of the sample at different working temperatures can be characterized.

[0029] In one specific embodiment, see Figure 2 The diagram shows the arrangement of the sample and testing equipment. The NCM ternary soft-pack lithium-ion battery is tightly wrapped with a 20 mm thick aerogel blanket, and then placed in an a×b×c mm container. 3 Inside an expanded polystyrene insulated container (where a=460mm, b=350mm, c=200mm), the small side of the battery serves as the insulation surface, with only two main surfaces (210mm×160mm) retained as the heating and cooling surfaces. A flexible Kapton heater (210mm×160mm×0.2mm) is selected and tightly fitted to one of the main surfaces of the battery as the heating surface. A Z2014 type heat flux sensor (53.5mm×10mm×0.3mm, accuracy ±2%) and a T-type thermocouple (0.26mm wire diameter, accuracy ±0.4%) are installed at the center of the heating and cooling surfaces, respectively. The heat flux sensors are designated HFS1 (heating surface) and HFS2 (cooling surface), and the thermocouples are designated TC1 (heating surface) and TC2 (cooling surface). HFS1, HFS2, TC1, and TC2 are all connected to a HIOKIALR8432 type heat flux meter, and the equipment is debugged to ensure normal data acquisition.

[0030] Place the battery and insulating container with the equipment in the thermostat chamber, set the initial temperature of the chamber to -30℃, and let it stand until the battery temperature and the chamber temperature reach thermal equilibrium.

[0031] In one specific embodiment, temperature change and external heat flux of the battery: During testing, appropriate heating power should be provided to ensure a temperature difference of 5 to 15°C between the heating and cooling surfaces, thereby reducing measurement errors in the batteries. After several trials, an input power of 60 W was ultimately selected for the two large pouch batteries. In this test, the initial temperature of the constant temperature chamber was -30°C, and measurements began when the batteries were at room temperature equivalent to ambient temperature. Furthermore, when the heater was powered on, temperature and heat flux were recorded simultaneously, with the heating time recorded as 0. The heater was powered off when the battery temperature approached 65°C.

[0032] Based on NCM capsule batteries, Figure 3 The changes in temperature and heat flux during heating are shown (actual heating power is expressed as a time equation: -4.97 × 10⁻⁶). -4 t + 61.78, W), where T 1 and T 2 represents the temperature of the heating surface and the cooling surface, respectively; T avg This indicates the average temperature of the battery. q 1 and q 2 represents the heat flux of the heating surface and the cooling surface, respectively.

[0033] Figure 3 Figure 3(a) shows that the NCM battery temperature increases over time during heating, reaching 60 °C after approximately 1025 seconds. Note that the slope of the battery temperature increase decreases over time due to heat flow (heat loss). Figure 3 In 3(b), the heat flux gradually increases with time, among which q 1 (Heat flux of the heating surface) is always greater than q 2 (Heat flux to the cooling surface), which is due to the higher temperature of the heating surface.

[0034] In one specific embodiment, the battery thermal parameters are calculated based on... Figure 3 The data shown in 3(a) is as follows: Figure 4 Figure 4(a) shows the calculated temperature difference between the heating and cooling surfaces of the NCM battery. T 1 - T 2) Figure 4 Figure 4(b) shows the calculated rate of temperature rise of the battery. dT rise / d t ).

[0035] Figure 4 (a) Display, T 1 - TThe temperature difference between points 2 and 2 increases rapidly from 0 to 84 seconds, then remains at almost the same level but increases slowly. This increase can be attributed to the difference in heat loss between the two main surfaces of the battery. Therefore, the battery temperature field reaches a quasi-steady state at 84 seconds. Figure 4 Figure 4(b) shows that the rate of temperature rise slows down over time, which can be attributed to heat loss. Therefore, the calculated temperature difference... T 1 - T 2 and the rate of temperature rise dT rise / d t Substituting these values ​​into the formula, the transverse thermal conductivity and specific heat of the NCM battery can be determined.

[0036] In one specific embodiment, Figure 5 The derived curves show the relationship between the cross-sectional thermal conductivity of the battery and the operating temperature between -20 and 60 degrees Celsius.

[0037] Figure 5 The maximum test deviations for both NCM and LFP batteries were less than 1.5%. Furthermore, the derived normal thermal conductivity of these two pouch cells increased linearly with operating temperature from -20°C to 60°C, with expansion rates of 4.8% and 6.1%, respectively. The positive and negative materials in the batteries exhibit typical porous structures, thus the heat transfer mechanism can be explained by a combination of convective and conductive heat transfer. Additionally, electrolyte diffusion increases with increasing temperature, which may accelerate heat transfer within the battery. Simultaneously, inorganic, non-metallic, and compound metals, as the main components of the battery, play a positive role in improving the battery's thermal conductivity with increasing temperature. (Comparison) Figure 5 5(a) and Figure 5 Figure 5(b) shows that the thermal conductivity of NCM cells is higher than that of LFP cells (on average 10.5% higher). This may be due to differences in battery manufacturing processes, as well as variations in electrode material, electrolyte content, lamination gap, and separator thickness. Therefore, different types of pouch cells exhibit differences in normal thermal conductivity. A key potential application of these measured transplanar thermal conductivity measurements is their potential use in designing BTM systems for pouch cells employing both L / NCM and LFP cathode materials.

[0038] For the system or system embodiments, since they are basically similar to the method embodiments, the description is relatively simple, and relevant details can be found in the description of the method embodiments. The systems and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0039] 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 measuring the normal thermal conductivity of a pouch battery, characterized in that, Includes the following steps: S1. Sample pretreatment: Wrap the soft-pack battery with thermal insulation material, and then place it in a constant temperature environment, taking the small side of the battery as the heat insulation surface. S2. Test setup: A flexible heater is attached to the main surface of the battery. A heat flow sensor and a thermocouple are placed at the center of the heating and cooling surfaces, respectively. The heat flow sensor is used to collect heat flux, and the thermocouple is used to collect temperature. S3. Creating a constant temperature environment: Place the prepared test device in the thermostat chamber and set the initial temperature of the chamber so that the sample temperature is consistent with the chamber temperature. S4. Thermal excitation loading and data acquisition: Power the flexible heater with a regulated power supply to heat the main surface of the sample, i.e. the heating surface, at a constant power. At the same time, record the heating time, heat flux and temperature change of the heating and cooling surfaces until the set upper limit temperature threshold is reached and then stop heating. S5. Quasi-steady state determination: Based on the collected temperature data, calculate the temperature difference between the battery heating surface and the cooling surface. When the temperature difference tends to be constant within a preset time, it is determined that the battery temperature field has reached a quasi-steady state. S6. Calculation of Normal Thermal Conductivity: Based on Fourier's law of thermal conductivity and the law of conservation of energy, a theoretical model for the normal thermal conductivity of a battery is derived. Substituting the temperature difference, heat flux, and battery size under quasi-steady-state conditions into the model, the normal thermal conductivity is calculated as follows: In the formula: For normal thermal conductivity, L For battery thickness, For a constant heating power density of the heating surface, For the heat flux of the heating surface, For the heat flux of the cooling surface, A The surface area of ​​the heating film. The temperature of the heating surface. This refers to the temperature of the cooling surface.

2. The method for measuring the normal thermal conductivity of a soft-pack battery according to claim 1, characterized in that, The insulation material in S1 that comes into contact with the battery is aerogel felt, which is then covered with expanded polystyrene insulation material. The thickness of the aerogel felt is 20 mm.

3. The method for measuring the normal thermal conductivity of a soft-pack battery according to claim 2, characterized in that, The sample in S1 is a soft-pack lithium-ion battery, including ternary soft-pack lithium-ion batteries, lithium iron phosphate soft-pack lithium-ion batteries, and other types of square soft-pack batteries.

4. The method for measuring the normal thermal conductivity of a soft-pack battery according to claim 3, characterized in that, The S2 heat flow sensor has an accuracy of ±2%, the thermocouple is a type T thermocouple with a wire diameter of 0.26 mm and an accuracy of ±0.4%, and the size of the flexible heater matches the size of the main surface of the sample.

5. The method for measuring the normal thermal conductivity of a soft-pack battery according to claim 4, characterized in that, The input power of the flexible heater in S4 meets the requirement that the temperature difference between the heating and cooling surfaces of the sample is 5~15 ℃, and the upper temperature threshold is 65 ℃. The heating power in S4 is constant, and the actual output power of the flexible heater is monitored and recorded in real time through a DC power supply.

6. The method for measuring the normal thermal conductivity of a soft-pack battery according to claim 5, characterized in that, The preset time in S5 is 84 seconds. When the temperature difference tends to be constant after 84 seconds, the current time node is determined as the quasi-steady-state starting point of the temperature field, and the temperature and heat flux data after the current node are used for calculation.

7. The method for measuring the normal thermal conductivity of a soft-pack battery according to claim 6, characterized in that, It also includes S7, method verification: using a quartz glass block with known thermal parameters as a standard sample, and measuring according to S1~S6. When the measurement error of the thermal conductivity of the standard sample is ≤6.5%, the measurement method is deemed valid.

8. A method for measuring the normal thermal conductivity of a pouch battery according to any one of claims 1-7, characterized in that, It can measure the normal thermal conductivity of a sample in the temperature range of -20 ~ 60 ℃, and by changing the initial temperature, it can characterize the normal thermal conductivity of the sample at different working temperatures.

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