Lithium-ion batteries have become the main source of power for electric vehicles and hybrid electric vehicles due to their high energy and high power density, while high power also brings more heat. However, lithium batteries are very sensitive to temperature, too high or too low. The operating temperature will affect the performance and life of the battery. An effective and compact thermal management system (TMS) is required to manage its extreme temperature rise. Active TMS and passive TMS are two popular thermal management techniques.
Active thermal management requires energy-driven thermal management systems, including: forced air cooling, water cooling, heat pipe cooling, etc., are common cooling methods. Passive thermal management is marked by non-consumption of energy, including natural cooling, phase change Material cooling and so on.
Active thermal management
Tran et al. designed a heat pipe module for thermal management of lithium-ion batteries. They found that heat pipes with different ventilation methods proved to be effective thermal management solutions for HEV batteries. Greco et al. Developed a heat pipe model. A one-dimensional transient model combined with thermal circuitry. They demonstrate that by using a one-dimensional transient model, the temperature of the lithium-ion battery drops from 52°C to 28°C. Mohammadian et al. embed the aluminum foam in the heat sink to cool the lithium ions. Batteries. They found that the surface temperature of lithium-ion batteries was significantly reduced after the aluminum foam was used inside the radiator compared to the case without foam aluminum. Zhao et al. used a liquid cooling cylinder to cool lithium-ion batteries. They used hydraulic cylinders. The surface temperature of 42 cylindrical batteries is kept below 40° C. Due to the addition of heat pumps, radiators, fan assemblies, etc., all of the above active cooling methods are expensive.
Passive thermal management
Passive thermal management (such as phase change materials) has received increasing attention in recent years because of its high efficiency, compactness, and light weight. Phase change materials (PCMs) store heat in the form of latent heat, and latent heat sources mainly include water, paraffin. And some other inorganic salts, etc. During the latent heat storage, the PCM undergoes a phase change from a solid to a liquid or a liquid to a gas at a nearly constant temperature. PCM is used in a TMS system and generally has the following requirements:
1) have suitable phase transition temperature, large latent heat of phase change, suitable thermal conductivity (generally large);
2) The phenomenon of smelting should not occur during the phase transition so as not to cause changes in the chemical composition of the phase change medium;
3) The reversibility of the phase transition is better, the degree of supercooling should be as small as possible, and the performance is stable;
4) Non-toxic, non-corrosive, non-polluting;
5) The use of safety, non-flammable, explosive or oxidative deterioration;
6) Faster crystallization rate and crystal growth rate.
7) Physical performance requirements: Low vapor pressure; Smaller volume expansion rate; Higher density;
8) Economic performance requirements: Easy to buy raw materials, cheaper prices.
This article mainly introduces a paper “Thermal management of lithium ion batteries using graphene coated nickel foam saturated with phase change materials” published by the Hong Kong University of Science and Technology, Abid Hussain et al., published in 'International Journal of Thermal Sciences' in 2018. The literature mainly introduces graphite. Epoxy coated foam nickel composite wax material PCM material superior characteristics in the application of power lithium battery thermal management system. With the discharge current at 1.7A, compared with nickel foam, paraffin wax, GcN, nickel foam + paraffin, and GcN + paraffin The effect of heat management, found that the use of foamed nickel compared to the use of saturated graphene coated foam nickel than the mere use of nickel foam, the battery surface temperature rise decreased by 17%.
1 Field Overview
Ordinary PCM has a very low thermal conductivity (0.1-0.3W/(m·K)). The heat storage rate is affected by the low thermal conductivity of PCM. Many techniques have been mentioned in the literature to improve the thermal conductivity of PCM. Goli et al. used graphene composites to improve the thermal conductivity of pure PCMs. They found that graphene/paraffin composites had a thermal conductivity of 45 W/(mK), whereas pure graphite had a thermal conductivity of 0.2 W/(mK). It was also observed that at 5 A discharge current, the graphene/paraffin composite lithium ion battery temperature rises 16° C. without graphene/paraffin composite temperature rise of 37° C. Kizilel et al. use a graphite matrix to increase Thermal conductivity. They observed that the thermal conductivity of paraffin graphite is about 17 W/(mK). The mixed PCM helps the lithium ion battery to be uniform in temperature under normal and high pressure conditions. Aadmi et al. added solid paraffin to the metal tube body. The thermal conductivity of the epoxy resin was increased by 3-4 times. They found that higher energy storage capacity and lower temperature rise can be obtained by increasing the wax content in the composite material.
Metallic foams have also proven to be a viable option for enhancing PCM thermal conductivity. High porosity, good thermophysical properties and mechanical strength are the salient features of metal foams. Li et al. used foamed copper paraffin composites to study 10 Ah lithium-ion batteries. The performance of the thermal management systems of the group. They compared the results with two models: natural air convection and pure paraffin. At a discharge rate of 1C, a foamed copper composite was used as a thermal management material to compare the air convection pattern with the paraffin wax pattern. The battery surface temperature was 29% and 12% lower respectively. Hussain et al. used a foamed nickel composite to experimentally study the cell surface temperature of a 3.4Ah lithium-ion battery pack. They found that at a 2C discharge rate, the battery surface temperature was lower than that of natural air and pure Paraffin model dropped by 31% and 24%, respectively. After Samimi et al. used carbon fiber paraffin composites, the cell surface temperature dropped by 15°C. Compared to pure paraffin, thermal conductivity of composites increased by 81-273%. Sabbah et al. Graphite was used to improve the thermal conductivity of paraffin. They used electric heaters as batteries and found that due to the use of graphite-PCM composites, Surface temperature is reduced 5%. Khateeb et al aluminum foam used to improve the thermal conductivity of paraffin. They found that the surface temperature of the battery 13.2Ah 5% lower than normal waxes.
In previous studies, the thermal management of lithium-ion batteries was mainly performed using graphene-graphite composites or metal foams (copper, nickel or aluminum)/graphite composites. The thermal conductivity of graphene was very high (~2000- 3000W/ (mK) ). The thermal conductivity of paraffin wax has been improved by foam nickel immersed in foamed nickel and graphene coatings.
The problem is that foamed nickel can only increase the thermal conductivity by a factor of six, and the thermomechanical properties (such as tensile strength and compressive strength) of graphite-polystyrene composites become weak at high temperatures. Here, we report that Graphene-coated foam nickel is used as a thermal management system for lithium-ion batteries.
2 graphene coated nickel foam as a lithium battery thermal management system material overview
In order to solve the above problems, in this study, the thermal management of lithium-ion batteries employs new thermal management materials (graphene, a combination of metal (nickel) foam and paraffin). The advantages of using nickel are many: corrosion resistance, high ratio Strength and Toughness. The mechanical properties of nickel can be enhanced by reinforcing the nickel with fibers/particles. Carbon atoms can be easily dissolved in nickel because of their high solubility in nickel and the nickel surface can be easily preformed. The pattern can therefore be precisely patterned to a preferred geometric shape. Foam nickel is suitable for graphene synthesis. Graphene exhibits excellent compatibility with a range of porous materials. Graphene-based composites have advantages over composites themselves. The mechanical properties, and have a lower coefficient of thermal expansion. Zhao et al. reported that the Young's modulus and mechanical properties of graphene composites (polyvinyl alcohol and graphene nanosheets) were improved by about 10 times and 150%. Growth of graphene on foamed nickel will make GcN foam harder (with 0.05g/L addition of graphene, Ni foam hardness is 1.2 times higher). Cycle performance of GcN foam Excellent (capacity retention rate of 98% after 10,000 cycles at 3mA/cm).
In this study, the thermal management of lithium-ion batteries was accomplished by using a novel material, saturated graphene-coated nickel foam (GcN). The growth of graphene coated on nickel foam was performed using chemical vapor deposition. Obtained. The thermal conductivity of the paraffin wax added to the GcN foam is 23 times higher than that of the pure paraffin wax. The paraffin wax is used as a phase change material (PCM). Compared with pure paraffin, paraffin of GcN foam is added and its melting temperature is increased. The solidification temperature is reduced. Compared with pure paraffin, the latent heat and specific heat of paraffin added with GcN foam are reduced by 30% and 34%, respectively. The application effects of 5 kinds of materials in thermal management of lithium battery are compared respectively: Foam nickel, solid Paraffin, GcN, Foam Nickel + Paraffin, and GcN + Paraffin. Graphite-coated foamed nickel saturated with PCM reduced the surface temperature of the cell by 17% compared to using foamed nickel at a discharge current of 1.7A.
3 conclusions
A graphene-coated foamed nickel is prepared using a chemical preparation vapor deposition technique. The thickness of the foamed nickel graphene layer is 1-2 nm. Paraffin is immersed in the graphene-coated foamed nickel as a phase change material. Paraffin-impregnated graphene The thermal properties of the coated nickel foam and its application in the thermal management of lithium-ion batteries are summarized below.
1) The laser pulse method was used to measure the thermal conductivity of the saturated graphene-coated foamed nickel paraffin composite and the foamed nickel paraffin composite. The paraffin penetrated deep into the support material. The results show that the graphene coated nickel foam will have the thermal conductivity of the pure paraffin. Increased by 23 times, while foamed nickel increased the thermal conductivity of pure paraffin 6 times.
2) Observe the changes of melting temperature and freezing temperature of saturated graphene-coated foamed nickel paraffin composites and foamed nickel paraffin composites, and compare them with pure paraffin waxes. Composite materials (foam nickel/graphene coated with foamed paraffin wax The melting temperature and freezing temperature of foamed nickel) increase and decrease, respectively; this is due to the interaction of foamed nickel with paraffin.
3) Compared with the latent heat of pure paraffin, the latent heat of saturated graphene-coated foamed nickel paraffin composites is reduced by 30%. This is due to the decrease of the mass fraction of paraffin in saturated graphene-coated foamed nickel paraffin composites. The reduction in paraffin wax mass is due to the presence of a small cavity in the metal foam. The intrinsic heat capacity of the foamed nickel paraffin composite is respectively 16% and 12% lower than that of pure paraffin wax in solid and liquid state, while saturated graphene coated foamed nickel paraffin The specific heat capacity of the composite is 35% and 34% less than that of pure paraffin in solid and liquid states, respectively. The reason is that the heat capacity of the metal skeleton (foam nickel and graphene-coated nickel foam) is smaller than that of pure PCM.
4) Finally, the study included the development of materials for applications (ie, lithium-ion battery thermal management). Four additional thermal management materials were also studied and compared: nickel foam, graphene-coated nickel foam, and paraffin wax. Foamed nickel paraffin. At 1.7A discharge current, the use of saturated graphene-coated foamed nickel has a 17% reduction in surface temperature rise compared to pure nickel foam.
