Thermophysical characterization of innovative and recyclable composites, being developed and considered for battery boxes mass production

This paper describes the results of preliminary thermophysical characterization carried out on recyclable & biobased Fibre Metal Laminates (FML) for lighter, more sustainable & safer battery box production, with higher fire resistance. These FMLs are designed to be suitable for the low and medium end automotive. In particular, the specific heat and the thermal conductivity of four different types of FMLs were analysed and assessed depending on temperature. The results show increasing trends of specific heat by increasing temperatures and thermal conductivity values increasing both with temperature and the density of the samples. Further studies will be carried out both to optimise the composition of the FMLs and to provide extensive assessment of the thermal and mechanical properties of the solutions so as to guarantee high thermal and mechanical performances and durability of battery boxes for electric vehicles.


Introduction
The transport sector is responsible for almost 28% of total carbon dioxide (CO2) emissions, while road transport is responsible for more than 70% of transport sector emissions [1].Electric vehicles (EVs) represent a promising way to reduce the greenhouse effect and particulate emissions and offer several advantages as follows: • Zero emissions • Ease of maintenance • Accessibility in all urban areas • Healthiness in high traffic areas.
According to the International Energy Agency -IEA, to stay on track and achieve net zero CO2 emissions by 2050, the percentage of EVs sold must reach roughly 60% by 2030.By the end of 2021, the number of electric vehicles on the road will have surpassed 16.5 million.By 2030, the global electric car stock will have increased to about 350 million vehicles, but further expansion will be dependent on efforts to diversify battery production and crucial mineral sources in order to decrease the risks of supply bottlenecks and higher pricing.The battery and relevant boxes are key components for EVs.The use of enhanced lithium-ion batteries with improved efficiency, durability, and cheaper cost provides an effective basis for EVs' recent success and an increase in popularity.Improved energy and power performance, longer cycle and reduced prices are resulting in EVs with longer electric range and implementation of incentives that are appealing to customers.
In order to ensure high vehicle performance, not only the batteries but also the housing boxes for the battery packs should be optimised, to guarantee The paper describes the results of the preliminary thermophysical characterization carried out on four different types of FML solutions focusing on the relevant thermal properties which could allow the wide exploitation of these materials.

Solutions Developed
Four different solutions were developed, with different lay-up structures of the FML solutions.The FMLs were obtained by associating two types of prepregs (basalt reinforced Polyfurfuryl alcohol, 630 gsm basalt fabric, procured from Basaltex, www.basaltex.com,and glass reinforced Crosspreg ® , 400 gsm glass procured from Crossfire, www.crossfire-srl.com)with aluminium sheets and the final structure of the four solutions is summarised in Table 1.In terms of sustainability, the strength point of PFA is being water based and biobased, while Crossfire resins and interesting because of closed loop recyclability and the fact they are VOC (Volatile Organic Compound) and solvent free.Another advantage of Crossfire resin is high adhesion on aluminium, which does not need special cleaning of pretreatments.

Experimental Method
The thermal behaviour of the solutions was analysed by means of two different techniques, namely Differential Scanning Calorimetry (DSC) and Heat Flow Meter.Differential Scanning Calorimetry (DSC) was used to evaluate the specific heat depending on temperature as well as to substantiate the thermal history of the composites [2] and determine suitable temperatures for effective analysis of the thermal conductivity.
As regards the DSC, differential scanning calorimetry was carried out by using the DSC 250 instrument (TA Instruments).The instrument was previously calibrated in accordance within the expected temperature ranges up to 300 °C.The thermal analysis was conducted between 0 °C and 200 °C.Differential scanning calorimetry records, as a function of temperature, the difference in heat flow that diffuses between the sample holder, the reference sample holder and the equipment test unit (heat flow DSC).The specific heat of the material is measured by using Equation ( 1): Where dH/dt = heat flux signal measured by DSC; Cp = Specific heat of the sample per unit weight; dT/dt =heating rate; f(T,t) = heat flow depending on the time at an absolute temperature (kinetic).
With reference to the heat flow meter apparatus -DTC 300 -TA Instruments allowed determining the thermal resistance and thermal conductivity of the four types of solutions.The thermophysical parameters were determined at the specific temperatures of 80 °C and 150 °C, where the developed solutions are stable and do not experience any phase transitions as confirmed by Differential Scanning Calorimetry analysis previously carried out on the test samples.The heat flow meter technique is based on a calibration procedure employing five reference samples with known and certified thermal properties.A sample of the material to be tested is held under a reproducible compressive load between two polished metal surfaces, each kept at a different and constant temperature.The lower contact surface is part of a calibrated heat flux transducer.As heat flows from the upper surface through the sample to the lower surface, an axial temperature gradient is established in the stack.By measuring the temperature difference across the sample and the output from the heat flux transducer, thermal conductivity of the sample can be determined when the thickness is known (see eq.2).

𝑅 = 𝑠 𝜆
Where: Rs: Thermal Resistance (m 2 K/W); s=Thickness of the sample; λ= Thermal conductivity of the sample (W/mK) Additionally, an uncertainty analysis on the thermal conductivity measurements was carried out after the experimental tests, by means of a statistical analysis model [3].

Results and Discussion
With reference to the analysis carried out through DSC, the samples were obtained by machining them into three samples for each type of solution developed.In particular, only the internal part of the FML, i.e. by removing the aluminium layers, was analysed in the temperature range between 0 °C and 200 °C, since the thermal behaviour of the aluminium is stable in the above-mentioned temperature range as already extensively reported in the literature [4].The masses of the samples of each FML solution are of about 15 mg and the specific test parameters are reported in Table 2. Figure 1 shows the average values of values of the three specimens for each type of solution.In particular, the average values of the specific heat values vary between 0,66 J/g K and 1,16 J/g K.The four trends are almost similar and are consistent with literature findings on FMLs [5].However, some instabilities occur around 60 °C that may be due to any phase transitions in the core part of the FML.
The samples for the assessment of the thermal conductivity at 80 °C and 150 °C were obtained by machining the slabs into three circular shaped samples each type of FML as shown in Figure 2. The samples have a diameter of about 50,8 mm and a thickness of about 3 mm as reported in Tables 3-6.
The results of the experimental analysis in Figure 3-5 and Tables 2-5, Type C shows the highest values of thermal conductivity at 80 °C that are closely connected to the highest values of the density.The difference in the thermal conductivity average values at 80 °C for Type A and Type C is about 17%.This difference reduces by to about 11% at 150 °C.The difference in average conductivity values between Type D and Type E is about 3 % at 80 °C, and then decreases up to 2 % at 150 °C.

Conclusion
Thermophysical characterization was carried out with different technologies in order to assess the specific heat and thermal conductivity of four different recyclable & biobased FMLs for lighter, more sustainable & safer battery box production, with higher fire resistance.The specific heat is similar for the four solutions.As regards type A and Type C, the two specific heat trends overlap at low temperatures and show a maximum difference of about 4% up to 100 °C.At higher temperatures the two trends diverge with a maximum difference in the average specific heat values of about 8%.As regards Type D and E, the two specific heat trends overlap at low temperatures and show a maximum difference of about 5% up to 100 °C.At higher temperatures the two trends diverge with a maximum difference in the average specific heat values of about 7%.With reference to thermal conductivity, Type C shows the highest values of thermal conductivity at 80 °C that are closely connected to the highest values of the density.The difference in the thermal conductivity average values at 80 °C is about 17%, when Type A and Type C are considered.This difference reduces to about 11% at 150 °C.The difference in average thermal conductivity values between Type D and Type E is about 3 % at 80 °C, and then decreases up to 2 % at 150 °C.Further studies will be conducted to optimise both the composition and properties of durable FMLs.

Figure 1 .
Figure 1.Specific heat trends for the four solutions

Figure 2 .Figure 3 .
Figure 2. Samples employed for the experimental analysis | Diameter of the samples 50.8 mm ± 0.25 mm

Figure 4 .
Figure 4. Thermal conductivity vs density assessed at 80 °C for the four solutions.

Figure 5 .
Figure 5. Thermal conductivity vs density assessed at 150 °C for the four solutions.

Table 4 .
Geometrical and thermal properties of the three samples - Programme was designed to meet the urgent need of efficient and affordable fire-resistant battery box solutions for the automotive electrification.The main aim of FENICE consists in developing recyclable & biobased Fibre Metal Laminates (FML) for lighter, more sustainable & safer battery box production, with higher fire resistance.These FMLs are designed to be suitable for the low and medium end automotive, thus putting a specific focus on mass production, which is the target to be addressed to alleviate the ongoing crisis of raw materials and energy procurement, and related C-footprint and climate change.
• Recyclability and reusability at end-of-life.To this end, the European Project -FENICE -FIRE RESISTANT ENVIRONMENTALLY FRIENDLY COMPOSITES (https://www.fenice-composites.eu/)-in the framework of the EIT Raw material

Table 1 .
Typologies of the solutions developed

Table 2 .
Characteristic test parameters employed for the experimental analysis through DSC

Table 3 .
Geometrical and thermal properties of the three samples -Type A

Table 5 .
Geometrical and thermal properties of the three samples -Type D

Table 6 .
Geometrical and thermal properties of the three samples -Type E