Thermal Performance Evaluation of lithium ion batteries: Study on test methods of thermal conductivity of battery materials

1. Raise questions

Lithium ion batteries are used for energy conversion and storage in various applications, including consumer electronic products, electric vehicles, aerospace systems, etc. Figure 1-1 shows the typical structure of lithium ion battery. Lithium ion battery mainly includes electrode material, electrolyte material, diaphragm material, battery pile and heat pipe tube high thermal conductivity phase change composite material.

Figure 1-1 structure diagram of lithium ion battery

As one of the important Thermophysical performance parameters of battery materials, thermal conductivity coefficient seriously affects various characteristics of lithium ion batteries. However, lithium ion batteries will face different boundary conditions of electricity, heat, force and mass in the process of use, which makes accurate testing of thermal conductivity of battery materials face severe challenges in the following aspects:

(1) lithium ion battery materials often involve energy-containing and energy-storing materials. Under different boundary conditions, such as the process of charging and discharging will be accompanied by heat generation and even pyrolysis, phase change materials are also involved in the battery thermal management system, which requires that the thermal conductivity coefficient should be measured simultaneously in these electrochemical and thermochemical processes, this is more complicated than the previous thermal conductivity testing technology in the process of pure Thermophysics changes.(2) There are many methods for testing thermal conductivity, but in view of the complex characteristics and requirements of lithium ion battery materials, reasonable testing methods need to be found first to ensure the accuracy of measurement results, this is especially important for lithium ion battery materials and battery thermal management.(3) due to the numerous environmental conditions involved in the thermal conductivity test of lithium ion battery materials, many different thermal conductivity test methods and equipment will be involved. However, in practical engineering applications, we still hope to optimize the testing methods and develop new testing technologies, so as to realize the test methods and instruments and equipment with less consumption to meet the thermal conductivity test requirements of various lithium ion battery materials as much as possible.(4) Because lithium ion battery materials also involve other thermal performance parameters and characterization parameters, such as specific heat capacity and thermal runaway, in this way, it is required that the thermal conductivity coefficient test method and instrument can be integrated with other thermal performance parameter test instruments, so that the test instrument has multi-function and can realize the test of multiple parameters in one test instrument.

In view of the above problems and challenges, this paper first reviews the thermal conductivity testing technology of lithium ion battery materials in recent years, and then on the basis of analysis and research, A practical method suitable for measuring the thermal conductivity of lithium ion batteries and materials is proposed.

2. Review of test methods for thermal conductivity of battery materials

In terms of the material level of lithium ion battery, the main materials involved include electrode, electrolyte, membrane, electrode membrane heap and heat pipe tube high thermal conductivity phase change composite material.

In terms of material grade, electrode [1]-[4], electrolyte [5], membrane [6][7], measurement results of thermal conductivity and contact thermal resistance of electrode reactor [2][8] [9][10].

As shown in figure 2-1, the thermal conductivity in the thickness direction of the cathode sample has been measured by the protective heat flux meter method (ASTM E1530) [1][12], the cathode is made of polymer electrolyte with equal volume fraction and a mixture of active material and acetylene black. After measurement, the thermal conductivity of the composite material changes in the range of 0.2~0.5W/mK between 25 ~ 150℃. Because the cathode material is too thin, the multi-layer cathode material is superimposed to form a measurable sample with the thickness of 1~2mm, and the diameter of the sample is 25.4mm, the test pressure is 10psi to reduce the contact thermal resistance caused by multi-layer superposition.

Figure 2-1 Thermal conductivity test schematic diagram of protective heat flux meter method

As shown in Figure 2-2, the structure of the thermal conductivity measuring device for lithium ion battery electrode material thickness direction is shown [2].

Figure 2-2 thermal conductivity of lithium ion battery material in thickness direction

The device adopts steady-state thin heating plate method [13], the area of single layer material is 431 mm2, the thickness is 0.42mm, and the tested sample is in the form of multi-layer superposition. The flash method is also adopted to measure the thermal diffusion coefficient of the thin layer material of the multilayer lithium ion battery, and the thermal diffusion coefficient in different directions is obtained through different sampling directions of the laminated material.

Time domain thermal reflection (TDTR) technology has been used to measure the thermal conductivity of LiCoO2 film thickness direction [3], and the thickness of the sample is about 500nm, which measures the influence of lithium degree on the thermal conductivity. In the process of circulation, the thermal conductivity of LiCoO2 cathode measured in the original position indicates that during the process of lithium removal, the thermal conductivity decreases reversibly from 5.4W/mK to 4.7W/mK.

As shown in figure 2-3, the thermal conductivity of negative electrode (NE) material made of synthetic graphite of various particle sizes is determined by flash method [4][14], and the sample size is about 15mm in diameter, the thickness range is 1.1~9.5mm, and the experiment was conducted at room temperature RT,150 and 200°C.

Figure 2-3 measurement principle of laser flash method

Similarly, the thermal conductivity of polymer electrolyte is measured by the protective heat flux meter method shown in figure 1-1 [5], measuring the temperature difference in the thickness direction of the sample, which is used to calculate the total thermal resistance, the thermal conductivity in the thickness direction of the sample can be extracted from it. The polymer electrolyte thin film sample is prepared by scraper technology, and it is clamped between the top plate and bottom plate of thermal conductivity instrument, and then the temperature difference is measured. It is reported that the thermal conductivity varies from 0.12 to 0.22W/mK in the range of 25 ~ 150℃.

As shown in Figure 2-4, the in-plane direction thermal conductivity of diaphragm material has been measured by DC heating method [6]. The membrane sample was extracted from 26650 lithium ion battery in the level 100 dust-free room, and two thin titanium wires with very small distance were deposited on the membrane sample, one of which was used as heater, these two lines are both used for temperature measurement, and the ultra-fast measurement of the temperature of the two lines as a time function is used to determine the thermal performance of the diaphragm sample [15]. The in-plane direction thermal conductivity at room temperature is 0.5W/mK. When measured at 50℃, these values do not change significantly.

Figure 2-4 specific heat capacity of diaphragm material and in-plane direction heat conduction system

The thermal conductivity of the thickness direction and in-plane direction of the positive and negative electrode film materials and membrane materials have been measured using different steady-state methods [7], the experimental device is very similar to the previously used one-dimensional heat flux meter device [1]. The size of the sample is 30mm × 30mm, the thickness of the single layer membrane is in the range of 24~106um, and the measurement result range of thermal conductivity coefficient is 0.19~31W/mK.

As shown in figure 2-5, the heat diffusion coefficient of the thickness direction and in-plane direction of the electrode membrane heap composed of multilayer anode, diaphragm and cathode is measured by flash method [8], and the specific heat capacity is measured by differential scanning calorimeter, thus, the thermal conductivity in the thickness direction and in-plane direction of the electrode diaphragm pile is obtained. In addition, the electrode membrane reactor taken out from the new battery was circulated for 500 times at 45℃, and the influence of high temperature circulation on thermal conductivity was investigated.

Figure 2-5 (a) flash method to test thickness direction and in-plane

In addition to the above-mentioned report on thermal conductivity measurement, the adoption of constant heat flow method (ASTM D5470) is also reported. The contact thermal resistance of the electrode diaphragm reactor was measured at different pressures and temperatures [9][16]. As shown in Figure 2-6, during the test, the stacked layer of the tested electrode diaphragm is clamped between two copper blocks, and the total thermal resistance of the stack is measured. The battery membrane reactor includes copper anode coated with graphite, aluminum cathode coated with lithium cobalt, polyethylene/polypropylene membrane and electrolyte. The test temperature range is -20 ~ 50℃, and the pressure is 0 ~ 250psi. The main conclusions drawn from the test include: compared with the dry battery group, the contact thermal resistance of the wet battery pack is lower, and the temperature dependence of the stacked thermal resistance of the electrode membrane is weaker. However, the thermal resistance measured here is the total thermal resistance, which also includes the thermal resistance of the material itself, not just the contact thermal resistance between different materials of the battery. The contact thermal resistance between the electrode used and the copper rod has been measured, which has no special relationship with the in-situ operation of the battery.

Figure 2-6 measurement of electricity by constant heat flow method (ASTM D5470)

As shown in figure 2-7, in another work, the interface heat conduction between the cathode and the diaphragm was measured by the same constant heat flow method (ASTM D5470) [10]. The measurement results show that the thermal characteristics of lithium ion batteries depend to a large extent on the heat transfer through the cathode-diaphragm interface, not through the heat transfer of the batteries themselves. This interface thermal resistance accounts for about 88% of the total thermal resistance of the battery.

Figure 2-7 measurement of battery material contact thermal resistance by constant heat flow method

As shown in Figure 2-8, the mixed phase change material of graphene filler [11][17] was measured by transient plane heat source method, the thermal conductivity of paraffin phase change materials before and after adding graphene is 0.25W/mK and 45W/mK respectively.

Figure 2-8 test probe and measuring original by transient plane heat source method

For such thin film materials as lithium ion battery materials, another very effective method to measure their thermal conductivity is temperature wave method [18]. Although this method has been introduced for many years, its application is still less, but it will be an important and effective method in the future.

3. Characteristics of test methods

From the above summary, we can see that the thermal conductivity of battery materials adopts the following test methods:

(1) steady-state protection heat flux meter method: ASTM E1530;(2) steady-state heat protection plate method: ASTM C177;(3) time domain reflection method;(4) flash method: ASTM E1461;(5) steady-state heat flux meter method: ASTM C518;(6) constant heat flow method: ASTM D5470;(7) transient plane heat source method: ISO 22007-2;(8) temperature wave method: ISO 22007-3.

It can be seen from the several testing methods mentioned above that, different from the test of thermal conductivity of traditional materials, the test of thermal conductivity of lithium ion battery materials presents the following remarkable characteristics:

(1) thin film: lithium ion battery materials basically present the form of thin film, and what is involved is the typical test technology of thermal conductivity of thin film;

(2) specificity: the thin-film lithium ion battery material presents obvious specificity characteristics, and the thermal conductivity coefficient shows obvious differences in the thickness direction and in-plane direction, the test of thermal conductivity of lithium ion battery material is actually a test problem of thermal conductivity of amorphous film material;

(3) many test variables: another remarkable feature of the thermal conductivity test of lithium ion battery materials is that there are many test condition variables. In addition to testing at different traditional temperatures, it also needs to include other test conditions, such as different loading pressure, SOC charge, atmosphere, vibration, humidity and other conditions, and even need to be in the power-on state.

4. Analysis of test methods for thermal conductivity of battery materials

According to the above characteristics of thermal conductivity testing of lithium ion battery materials, the above various testing methods are analyzed to find out which testing methods are more suitable for testing lithium ion battery materials.

Throughout the above test methods, we divide them into steady state method and transient method for analysis.

4.1 Steady State method

The steady state method mainly includes: protective heat flow meter method, heat protection plate method, heat flow meter method and constant heat flow method.

The remarkable feature of steady-state method is that according to the classical fourier steady-state heat transfer law, a stable one-dimensional heat flow is formed in the test direction of the tested battery material thin film samples, the corresponding thermal conductivity and contact thermal resistance are measured by measuring the temperature and heat flux density under different conditions.

As a traditional method, steady state method is a test method developed on the basis of thermal properties of thicker block materials. It is very accurate and mature to test the thermal conductivity of larger size and thicker block samples, such as protective heat flow meter method, heat protection plate method and heat flow meter method. In order to test the battery film material, it is necessary to make samples after multi-layer superposition of the film material to meet the measurement accuracy requirements of steady state method, and this multi-layer superposition is bound to bring serious impact of contact thermal resistance.

In view of the limitations of the traditional steady state method on the thermal conductivity testing of thin film materials, the developed constant heat flow rule partially solves the testing problem. Through the unique surface temperature testing technology, it can be used to measure the thermal conductivity of the thin film with the thickness of 100 micrometers, which is very suitable for testing the battery pile composed of multi-layer films and the high thermal conductivity phase change composite material.

Although corresponding improvements have been made, any effort made on the steady-state method is to explore the potential of the steady-state method and further expand the lower limit of the test capability interval of the steady-state method, after all, the lower limit of test capability is still very limited, which is restricted by the steady state method itself, especially by the accuracy of surface temperature and thickness measurement, making this expansion space very limited and the effect difficult to guarantee.

In a word, for lithium ion battery materials, the steady state method which is suitable for the time being is ASTM D5470 constant heat flow method, which can measure thermal conductivity and thermal resistance. The sample size is moderate and it is suitable to load various boundary conditions.

4.2 Transient Method

Transient Method mainly includes time domain reflection method, flash method and transient plane heat source method.

Contrary to steady state method, transient method is a test method based on the dynamic response of sample materials to thermal excitation. The thinner the tested sample is, the faster the response to thermal excitation is, therefore, the core of transient method is to detect the change speed of physical quantities over time. At the same time, in the rapid response of the tested sample to thermal excitation, the influence of the surrounding environment and other boundary conditions becomes very small. Most importantly, with the development of technology, the dynamic response time of block samples (especially thin film materials) to thermal excitation no longer belongs to the category of rapid measurement before the current electronic detection technology, it is easy to quickly and accurately measure the thermal excitation response by using various current electronic technical means. On the other hand, it is aimed at the thermal performance test of materials. The transient method can aim at the thickness range (response time) of different tested samples. Electronic instruments and equipment with corresponding response frequency range are adopted to realize accurate measurement, while the testing capability of electronic instruments and equipment at present far exceeds the requirements of thermal performance testing of thin film materials. This is the biggest advantage of transient method itself, and it is also the main reason why the thermal performance testing instruments of thin film materials mostly adopt transient method in the market at present.

In a word, the transient method, as a non-contact measurement method, is very suitable for compacting thin film materials and measuring very thin samples. However, for such low-density thin film materials as lithium ion battery materials, many test problems will be encountered, porous thin film material samples need surface treatment to measure thermal conductivity, but surface treatment often brings penetration and changes the thermal performance of thin film samples. In addition, another obvious disadvantage of transient method is that it is difficult to load various corresponding boundary conditions on the tested sample for thermal conductivity measurement, such as pressure and power-on. However, the temperature wave rule in the transient method is an exception, which will be introduced in the next section.

5. Future vision: proposing new methods

It can be seen from the above analysis of the thermal conductivity testing methods of battery materials that the existing methods cannot solve the problems faced by the thermal conductivity testing of lithium ion battery materials mentioned at the beginning of this paper, new testing methods need to be researched and developed to meet the corresponding technical challenges.

Through our research, we believe that the method of combining the above-mentioned steady-state method and transient method will be an effective technical approach, and the specific combination form is the improved transient temperature wave method.

The temperature wave test method specified by ISO 22007-3 [18] is mainly used to determine the thermal diffusion coefficient of thin films and plastic plates in the whole thickness direction. Temperature wave method is a method to measure the thermal diffusion coefficient in the thickness direction of thin and flat samples by measuring the phase shift of temperature waves between the front and back surfaces of samples. Using a resistor that is splashed or contacted on the two surfaces of the sample, one serves as a heater and generates a temperature wave through ac joules heating, and the other serves as a thermometer to detect the temperature wave. The schematic diagram of the temperature wave measuring device is given in ISO 22007-3, as shown in figure 5-1.

Figure 5-1 schematic diagram of thermal diffusion coefficient measuring device by temperature wave method

As can be seen from the above description, the temperature wave measuring device includes micro heaters and temperature sensors facing each other, and samples are installed between them. A weak sinusoidal electric power signal is provided to the heater, generating a temperature wave on the surface of the sample. Temperature sensor is a high sensitivity resistance sensor, which uses a preamplifier to amplify weak signals before entering the phase-locked amplifier. The observed temperature signal is a mixture of excited temperature waves and background temperature signals, such as ambient temperature. In ac measurement, one advantage of locking amplification is that it can extract and analyze the change of only one specified frequency component in the signal to offset the influence of room temperature change (the main source of error) and noise components to achieve high sensitivity measurement. By limiting the actual applied temperature wave amplitude within 1℃ or less, convection and radiation can be effectively inhibited and the sample can be ensured to be hardly damaged. In addition, if a very small sensor size is used, the thermal diffusion coefficient in a smaller sample area can be identified.

In a word, the improved temperature wave method will have the following remarkable characteristics:

(1) in the aspects of sample clamping, thickness control and measurement, the temperature wave method is basically the same as the steady state method, which can load certain pressure and other test conditions on the sample during the measurement process. At the same time, the temperature wave method also has the advantages of non-contact transient method, which converts temperature and heat flow measurement into high-precision frequency and phase measurement, reduces errors, and can realize highly sensitive measurement.

(2) although the temperature wave test method specified by ISO 22007-3 is used to measure the thermal diffusion coefficient in the thickness direction of the film material, this method can also be used to measure the thermal diffusion coefficient in the inner direction of the film surface, the converted test method is the classical Angstrom-period thermal wave method [19].

(3) it can be seen from the temperature wave measurement principle shown in figure 5-1 that as long as the ac heating form is controlled to dc form, the temperature wave method becomes the traditional heat flux meter method, it can be used for the measurement of plate samples, that is to say, it can be used to measure the thermal diffusion coefficient and thermal conductivity coefficient of bags of various specifications and sizes and flaky lithium ion batteries.

(4) the more important feature is that the improved temperature wave method has a small structure and can be integrated with other thermal performance testing methods. This aspect will be introduced in subsequent reports.

To sum up, we choose and carry out the study of the improved temperature wave method, which can basically solve several problems faced in the test of lithium ion battery materials mentioned above in this paper, at the same time, it also gives consideration to the miniaturization, integration and low cost of testing instruments, which will be a direction for the development of thermal analysis instruments in the future.