Thermal conductivity and temperature dependence of soft pack lithium batteries-CSIP

According to the different packaging methods, lithium-ion batteries can be divided into soft pack, square and cylindrical. Among them, soft pack lithium batteries are closely related to our daily life and are widely used in

3C electronic products and new energy vehicles. In this paper, the thermal conductivity of soft pack lithium batteries is measured using the TCA 3DP-160 3D thermal property analyser and the variation of thermal

conductivity with cell temperature is investigated. The results show that the thermal conductivity increases narrowly with temperature in both the longitudinal and oriented directions of the battery.

The increasing energy density of electric vehicle battery packs to alleviate driving range and fast charging anxiety has led to a gradual increase in the size of lithium-ion battery cells. This increase in battery size

exacerbates the uneven distribution of battery temperature and accelerates the decay of battery system life. Therefore, it is important to take effective thermal management measures for the battery pack, which requires

engineers to have an accurate understanding of the thermal parameters of the battery.

Thermal simulation is a major development aid and verification tool in the design and development of thermal management of lithium batteries. Thermal conductivity is one of the most important thermal parameters

required for thermal simulation and directly affects the thermal characteristics of the battery [1]. Soft pack batteries are multi-layer composite structures consisting of aluminium plastic film, positive and negative

materials, diaphragm, collector and electrolyte, and both the cell facing and longitudinal thermal conductivity refer to their integrated thermal conductivity. As the thermal properties of the cell material and the composite

microstructure vary with temperature, this leads to a temperature dependence of the integrated thermal conductivity value of the cell. Therefore, it is important to obtain data on the temperature dependence of the

thermal conductivity of the battery in the normal operating temperature range of the battery to improve the accuracy and validity of thermal management simulations.

The main reason for this is the lack of a universal and reliable analytical test tool. This paper investigates this issue using a new instrument, the 3D thermal property analyser, to determine the longitudinal and oriented

thermal conductivity trends with temperature for NCM soft pack batteries.

Experimental section

1. Sample preparation

Sample: NCM soft pack lithium battery (65Ah, 100% SOC)

2. Experimental conditions

Experimental instrument: TCA 3DP-160 3D thermal property analyzer, BIC-400A battery isothermal calorimeter

Operating mode: Transmittance mode

Experimental temperature: 5°C, 10°C, 20°C, 30°C

Figure 1 (a) 3D thermal property analyser; (b) experimental soft pack battery sample; (c) 3D thermal property analyser thermal conductivity data inversion analysis process

3. Testing process

The BIC-400A isothermal calorimeter was used to determine the specific heat capacity of the battery at the set temperature, which was used as a preset parameter for the thermal conductivity test.

Figure 2 BIC-400A battery isothermal calorimeter

The battery is then placed in the centre of the test chamber of the 3D thermal property analyser, the sample information is filled in, the relevant experimental parameters are set and the test is started. The instrument

automatically controls the temperature to the preset temperature and automatically performs the process of thermal excitation, 3D thermal data inversion and data verification once the battery temperature has stabilised,

followed by the software giving the battery thermal conductivity kx and the longitudinal thermal conductivity ky directly. 4 parallel experiments are performed at each temperature point to eliminate chance errors.

Figure 3 (a) Schematic diagram of the 3D Thermal Property Analyser cell sample installation; (b) 3D Thermal Property Analyser operating software interface

Experimental results

1. Specific heat capacity

As shown in Table 1, the specific heat capacity of the sample cells increased gradually with temperature, which is in accordance with the conventional variation rule [2].

Table 1 Results of specific heat capacity of lithium battery at different temperatures

2. Thermal conductivity

The experimentally measured thermal conductivity is shown in Table 2 and Figure 4:

Table 2 Test results of thermal conductivity of sample lithium batteries at different temperatures

Figure 4: Thermal conductivity versus temperature for (a) facing and (b) longitudinal direction of lithium battery

From Table 2 and Figure 4, it can be seen that the repeatability of the thermal conductivity of the 3D thermal property analyser is good, with the exception of a slight deviation due to low temperature condensation at 5°C.

The relative standard deviation of kx and ky for the four experiments at all temperatures was kept within 4%. At the same time, it can be found that the longitudinal and oriented thermal conductivity of the sample lithium

batteries both increased slightly with temperature, and this result is consistent with that reported in the relevant literature [3-4].

Conclusion

The TCA 3DP-160 3D thermal property analyser can be used to measure the thermal conductivity of soft pack lithium batteries conveniently, efficiently and accurately, and to conduct temperature and other operating

conditions influence studies to help researchers optimise and improve the thermal management design of lithium batteries.